diff --git a/lib/lrama/option_parser.rb b/lib/lrama/option_parser.rb
index 5a15d59c..023fee64 100644
--- a/lib/lrama/option_parser.rb
+++ b/lib/lrama/option_parser.rb
@@ -97,6 +97,7 @@ def parse_by_option_parser(argv)
         o.on_tail '    lookaheads                       explicitly associate lookahead tokens to items'
         o.on_tail '    solved                           describe shift/reduce conflicts solving'
         o.on_tail '    counterexamples, cex             generate conflict counterexamples'
+        o.on_tail '    ielr                             show where IELR splits LALR states, with lookahead diffs and split reasons'
         o.on_tail '    rules                            list unused rules'
         o.on_tail '    terms                            list unused terminals'
         o.on_tail '    verbose                          report detailed internal state and analysis results'
@@ -141,7 +142,7 @@ def parse_by_option_parser(argv)
     end
 
     ALIASED_REPORTS = { cex: :counterexamples }.freeze #: Hash[Symbol, Symbol]
-    VALID_REPORTS = %i[states itemsets lookaheads solved counterexamples rules terms verbose].freeze #: Array[Symbol]
+    VALID_REPORTS = %i[states itemsets lookaheads solved counterexamples ielr rules terms verbose].freeze #: Array[Symbol]
 
     # @rbs (Array[String]) -> Hash[Symbol, bool]
     def validate_report(report)
diff --git a/lib/lrama/reporter.rb b/lib/lrama/reporter.rb
index ed25cc7f..6a452d4b 100644
--- a/lib/lrama/reporter.rb
+++ b/lib/lrama/reporter.rb
@@ -3,6 +3,7 @@
 
 require_relative 'reporter/conflicts'
 require_relative 'reporter/grammar'
+require_relative 'reporter/ielr'
 require_relative 'reporter/precedences'
 require_relative 'reporter/profile'
 require_relative 'reporter/rules'
@@ -21,6 +22,7 @@ def initialize(**options)
       @conflicts = Conflicts.new
       @precedences = Precedences.new
       @grammar = Grammar.new(**options)
+      @ielr = Ielr.new(**options)
       @states = States.new(**options)
     end
 
@@ -32,6 +34,7 @@ def report(io, states)
         report_duration(:report_conflicts) { @conflicts.report(io, states) }
         report_duration(:report_precedences) { @precedences.report(io, states) }
         report_duration(:report_grammar) { @grammar.report(io, states) }
+        report_duration(:report_ielr) { @ielr.report(io, states) }
         report_duration(:report_states) { @states.report(io, states, ielr: states.ielr_defined?) }
       end
     end
diff --git a/lib/lrama/reporter/ielr.rb b/lib/lrama/reporter/ielr.rb
new file mode 100644
index 00000000..d7ed7c0f
--- /dev/null
+++ b/lib/lrama/reporter/ielr.rb
@@ -0,0 +1,165 @@
+# rbs_inline: enabled
+# frozen_string_literal: true
+
+module Lrama
+  class Reporter
+    class Ielr
+      # @rbs (?ielr: bool, **bool _) -> void
+      def initialize(ielr: false, **_)
+        @enabled = ielr
+      end
+
+      # @rbs (IO io, Lrama::States states) -> void
+      def report(io, states)
+        return unless @enabled && states.ielr_defined?
+
+        groups = split_groups(states)
+        return if groups.empty?
+
+        @incoming_index = build_incoming_index(states)
+
+        io << "IELR State Splits\n\n"
+
+        groups.each do |core|
+          report_group(io, core)
+        end
+      ensure
+        @incoming_index = nil
+      end
+
+      private
+
+      # @rbs (Lrama::States states) -> Array[Lrama::State]
+      def split_groups(states)
+        states.states.select do |state|
+          !state.split_state? && state.ielr_isocores.size > 1
+        end
+      end
+
+      # @rbs (Lrama::States states) -> Hash[Lrama::State, Array[Lrama::State::Action::Shift | Lrama::State::Action::Goto]]
+      def build_incoming_index(states)
+        index = Hash.new { |h, k| h[k] = [] } #: Hash[Lrama::State, Array[Lrama::State::Action::Shift | Lrama::State::Action::Goto]]
+        states.states.each do |state|
+          state.transitions.each do |transition|
+            index[transition.to_state] << transition
+          end
+        end
+        index
+      end
+
+      # @rbs (IO io, Lrama::State core) -> void
+      def report_group(io, core)
+        variants = core.ielr_isocores.sort_by(&:id)
+
+        io << "    LALR state #{core.id} splits into IELR states #{variants.map(&:id).join(', ')}\n\n"
+        report_incoming_transitions(io, variants)
+        report_lookahead_differences(io, variants)
+        report_split_reasons(io, variants)
+        io << "\n"
+      end
+
+      # @rbs (IO io, Array[Lrama::State] variants) -> void
+      def report_incoming_transitions(io, variants)
+        io << "      Incoming transitions\n"
+
+        variants.each do |variant|
+          @incoming_index[variant]
+            .sort_by { |t| [t.from_state.id, t.next_sym.number] }
+            .each do |transition|
+              io << "        state #{transition.from_state.id} -- #{transition.next_sym.display_name} --> state #{variant.id} #{state_role(variant)}\n"
+            end
+        end
+
+        io << "\n"
+      end
+
+      # @rbs (IO io, Array[Lrama::State] variants) -> void
+      def report_lookahead_differences(io, variants)
+        differing_items = variants.first.kernels.select do |item|
+          variants.map { |state| lookahead_signature(state.item_lookahead_set[item]) }.uniq.size > 1
+        end
+        return if differing_items.empty?
+
+        io << "      Lookahead differences\n"
+
+        differing_items.each do |item|
+          io << "        #{item.display_name}\n"
+
+          variants.each do |state|
+            io << "          state #{state.id} #{state_role(state)}: #{format_lookaheads(state.item_lookahead_set[item])}\n"
+          end
+        end
+
+        io << "\n"
+      end
+
+      # @rbs (IO io, Array[Lrama::State] variants) -> void
+      def report_split_reasons(io, variants)
+        core = variants.first.lalr_isocore
+        different_annotations = [] #: Array[[Lrama::State::InadequacyAnnotation, Hash[Lrama::State, String]]]
+
+        core.annotation_list.each do |annotation|
+          labels_by_state = {} #: Hash[Lrama::State, String]
+          variants.each do |state|
+            labels_by_state[state] = dominant_actions(state, annotation)
+          end
+          next if labels_by_state.values.uniq.size <= 1
+
+          different_annotations << [annotation, labels_by_state]
+        end
+        return if different_annotations.empty?
+
+        io << "      Why it split\n"
+
+        different_annotations.each do |annotation, labels_by_state|
+          io << "        token #{annotation.token.display_name}\n"
+
+          variants.each do |state|
+            io << "          state #{state.id} #{state_role(state)}: #{labels_by_state[state]}\n"
+          end
+        end
+
+        io << "\n"
+      end
+
+      # @rbs (Array[Lrama::Grammar::Symbol] syms) -> Array[Integer]
+      def lookahead_signature(syms)
+        syms.map(&:number).sort
+      end
+
+      # @rbs (Array[Lrama::Grammar::Symbol] syms) -> String
+      def format_lookaheads(syms)
+        values = syms.sort_by(&:number).map(&:display_name)
+        "[#{values.join(', ')}]"
+      end
+
+      # @rbs (Lrama::State state, Lrama::State::InadequacyAnnotation annotation) -> String
+      def dominant_actions(state, annotation)
+        actions = annotation.dominant_contribution(state.item_lookahead_set)
+        return "no dominant action" if actions.nil? || actions.empty?
+
+        actions.map { |action| format_action(state, action) }.join(", ")
+      end
+
+      # @rbs (Lrama::State state, Lrama::State::Action::Shift | Lrama::State::Action::Reduce action) -> String
+      def format_action(state, action)
+        case action
+        when Lrama::State::Action::Shift
+          current_shift = state.term_transitions.find { |shift| shift.next_sym == action.next_sym }
+          destination = current_shift ? current_shift.to_state.id : action.to_state.id
+          "shift and go to state #{destination}"
+        when Lrama::State::Action::Reduce
+          rule = action.item.rule
+          "reduce using rule #{rule.id} (#{rule.lhs.display_name})"
+        else
+          raise "Unsupported action #{action.class}"
+        end
+      end
+
+      # @rbs (Lrama::State state) -> String
+      def state_role(state)
+        state.split_state? ? "[IELR split]" : "[LALR core]"
+      end
+    end
+  end
+end
diff --git a/sig/generated/lrama/reporter/ielr.rbs b/sig/generated/lrama/reporter/ielr.rbs
new file mode 100644
index 00000000..e675a4eb
--- /dev/null
+++ b/sig/generated/lrama/reporter/ielr.rbs
@@ -0,0 +1,48 @@
+# Generated from lib/lrama/reporter/ielr.rb with RBS::Inline
+
+module Lrama
+  class Reporter
+    class Ielr
+      # @rbs (?ielr: bool, **bool _) -> void
+      def initialize: (?ielr: bool, **bool _) -> void
+
+      # @rbs (IO io, Lrama::States states) -> void
+      def report: (IO io, Lrama::States states) -> void
+
+      private
+
+      # @rbs (Lrama::States states) -> Array[Lrama::State]
+      def split_groups: (Lrama::States states) -> Array[Lrama::State]
+
+      # @rbs (Lrama::States states) -> Hash[Lrama::State, Array[Lrama::State::Action::Shift | Lrama::State::Action::Goto]]
+      def build_incoming_index: (Lrama::States states) -> Hash[Lrama::State, Array[Lrama::State::Action::Shift | Lrama::State::Action::Goto]]
+
+      # @rbs (IO io, Lrama::State core) -> void
+      def report_group: (IO io, Lrama::State core) -> void
+
+      # @rbs (IO io, Array[Lrama::State] variants) -> void
+      def report_incoming_transitions: (IO io, Array[Lrama::State] variants) -> void
+
+      # @rbs (IO io, Array[Lrama::State] variants) -> void
+      def report_lookahead_differences: (IO io, Array[Lrama::State] variants) -> void
+
+      # @rbs (IO io, Array[Lrama::State] variants) -> void
+      def report_split_reasons: (IO io, Array[Lrama::State] variants) -> void
+
+      # @rbs (Array[Lrama::Grammar::Symbol] syms) -> Array[Integer]
+      def lookahead_signature: (Array[Lrama::Grammar::Symbol] syms) -> Array[Integer]
+
+      # @rbs (Array[Lrama::Grammar::Symbol] syms) -> String
+      def format_lookaheads: (Array[Lrama::Grammar::Symbol] syms) -> String
+
+      # @rbs (Lrama::State state, Lrama::State::InadequacyAnnotation annotation) -> String
+      def dominant_actions: (Lrama::State state, Lrama::State::InadequacyAnnotation annotation) -> String
+
+      # @rbs (Lrama::State state, Lrama::State::Action::Shift | Lrama::State::Action::Reduce action) -> String
+      def format_action: (Lrama::State state, Lrama::State::Action::Shift | Lrama::State::Action::Reduce action) -> String
+
+      # @rbs (Lrama::State state) -> String
+      def state_role: (Lrama::State state) -> String
+    end
+  end
+end
diff --git a/spec/lrama/option_parser_spec.rb b/spec/lrama/option_parser_spec.rb
index 7675c4da..9c324588 100644
--- a/spec/lrama/option_parser_spec.rb
+++ b/spec/lrama/option_parser_spec.rb
@@ -80,6 +80,7 @@
               lookaheads                       explicitly associate lookahead tokens to items
               solved                           describe shift/reduce conflicts solving
               counterexamples, cex             generate conflict counterexamples
+              ielr                             show where IELR splits LALR states, with lookahead diffs and split reasons
               rules                            list unused rules
               terms                            list unused terminals
               verbose                          report detailed internal state and analysis results
@@ -134,11 +135,19 @@
           expect(opts).to eq({
             grammar: true, states: true, itemsets: true,
             lookaheads: true, solved: true, counterexamples: true,
+            ielr: true,
             rules: true, terms: true, verbose: true
           })
         end
       end
 
+      context "when ielr is passed" do
+        it "returns option hash ielr flag enabled" do
+          opts = option_parser.send(:validate_report, ["ielr"])
+          expect(opts).to eq({grammar: true, ielr: true})
+        end
+      end
+
       context "when none is passed" do
         it "returns empty option hash" do
           opts = option_parser.send(:validate_report, ["none"])
diff --git a/spec/lrama/reporter/ielr_spec.rb b/spec/lrama/reporter/ielr_spec.rb
new file mode 100644
index 00000000..5b81875e
--- /dev/null
+++ b/spec/lrama/reporter/ielr_spec.rb
@@ -0,0 +1,94 @@
+# frozen_string_literal: true
+
+RSpec.describe Lrama::Reporter::Ielr do
+  describe "#report" do
+    it "reports LALR vs IELR split diagnostics" do
+      y = <<~INPUT
+        %{
+        // Prologue
+        %}
+
+        %define lr.type ielr
+
+        %token a
+        %token b
+        %token c
+
+        %%
+
+        S: S2
+         ;
+
+        S2: a A1 a
+          | a A2 b
+          | b A1 b
+          | b A2 a
+          ;
+
+        A1: c
+         ;
+
+        A2: c
+         ;
+
+        %%
+      INPUT
+
+      grammar = Lrama::Parser.new(y, "ielr_diff.y").parse
+      grammar.prepare
+      grammar.validate!
+      states = Lrama::States.new(grammar, Lrama::Tracer.new(Lrama::Logger.new))
+      states.compute
+      states.compute_ielr
+
+      io = StringIO.new
+      described_class.new(ielr: true).report(io, states)
+
+      expect(io.string).to eq(<<~STR)
+        IELR State Splits
+
+            LALR state 5 splits into IELR states 5, 15
+
+              Incoming transitions
+                state 1 -- c --> state 5 [LALR core]
+                state 2 -- c --> state 15 [IELR split]
+
+              Lookahead differences
+                c •  (rule 6)
+                  state 5 [LALR core]: [a]
+                  state 15 [IELR split]: [b]
+                c •  (rule 7)
+                  state 5 [LALR core]: [b]
+                  state 15 [IELR split]: [a]
+
+              Why it split
+                token a
+                  state 5 [LALR core]: reduce using rule 6 (A1)
+                  state 15 [IELR split]: reduce using rule 7 (A2)
+                token b
+                  state 5 [LALR core]: reduce using rule 7 (A2)
+                  state 15 [IELR split]: reduce using rule 6 (A1)
+
+
+      STR
+    end
+
+    it "reports shift destinations from the current split state" do
+      grammar = Lrama::Parser.new(File.read(fixture_path("integration/ielr.y")), fixture_path("integration/ielr.y")).parse
+      grammar.prepare
+      grammar.validate!
+      states = Lrama::States.new(grammar, Lrama::Tracer.new(Lrama::Logger.new))
+      states.compute
+      states.compute_ielr
+
+      io = StringIO.new
+      described_class.new(ielr: true).report(io, states)
+
+      expect(io.string).to include("state 19 [IELR split]: shift and go to state 8")
+      expect(io.string).to include("state 20 [IELR split]: shift and go to state 8")
+      expect(io.string).to include("state 21 [IELR split]: shift and go to state 17")
+      expect(io.string).not_to include("state 19 [IELR split]: shift and go to state 17")
+      expect(io.string).not_to include("state 20 [IELR split]: shift and go to state 17")
+    end
+  end
+end