diff --git a/src/FastExpressionCompiler.LightExpression/FlatExpression.cs b/src/FastExpressionCompiler.LightExpression/FlatExpression.cs
index 11c12e74..b458e288 100644
--- a/src/FastExpressionCompiler.LightExpression/FlatExpression.cs
+++ b/src/FastExpressionCompiler.LightExpression/FlatExpression.cs
@@ -45,6 +45,23 @@ public enum ExprNodeKind : byte
     UInt16Pair,
 }
 
+/// <summary>Maps a lambda node to a parameter identity used from an outer scope and therefore captured in closure.</summary>
+[StructLayout(LayoutKind.Sequential, Pack = 2)]
+public readonly struct LambdaClosureParameterUsage
+{
+    /// <summary>The lambda node index containing the parameter usage.</summary>
+    public readonly short LambdaNodeIndex;
+
+    /// <summary>The parameter identity id (<see cref="ExprNode.ChildIdx"/>) referenced from outer scope.</summary>
+    public readonly short ParameterId;
+
+    public LambdaClosureParameterUsage(short lambdaNodeIndex, short parameterId)
+    {
+        LambdaNodeIndex = lambdaNodeIndex;
+        ParameterId = parameterId;
+    }
+}
+
 /// <summary>Stores one flat expression node plus its intrusive child-link metadata in 24 bytes on 64-bit runtimes.</summary>
 /// <remarks>
 /// Layout (64-bit): Type(8) | Obj(8) | _meta(4) | _data(4) = 24 bytes.
@@ -197,6 +214,12 @@ public struct ExprTree
     /// enabling callers to locate all try regions without a full tree traversal.</summary>
     public SmallList<int, Stack16<int>, NoArrayPool<int>> TryCatchNodes;
 
+    /// <summary>Gets or sets mappings of lambda-node index to captured parameter id for nested-lambda closures.
+    /// Populated by <see cref="ExprTree.FromExpression"/> while flattening System.Linq lambdas;
+    /// each entry means that the lambda body references a parameter that is not declared as that lambda parameter
+    /// and not declared as a local block/catch variable in that lambda body scope.</summary>
+    public SmallList<LambdaClosureParameterUsage, Stack16<LambdaClosureParameterUsage>, NoArrayPool<LambdaClosureParameterUsage>> LambdaClosureParameterUsages;
+
     /// <summary>Adds a parameter node and returns its index.</summary>
     public int Parameter(Type type, string name = null)
     {
@@ -900,6 +923,7 @@ private int AddExpression(SysExpr expression)
                             children.Add(AddExpression(lambda.Parameters[i]));
                         var lambdaIndex = _tree.AddRawExpressionNode(expression.Type, null, expression.NodeType, children);
                         _tree.LambdaNodes.Add(lambdaIndex);
+                        CollectLambdaClosureParameterUsages(lambda, lambdaIndex);
                         return lambdaIndex;
                     }
                 case ExpressionType.Block:
@@ -1140,6 +1164,91 @@ private int AddExpression(SysExpr expression)
             }
         }
 
+        private void CollectLambdaClosureParameterUsages(System.Linq.Expressions.LambdaExpression lambda, int lambdaNodeIndex)
+        {
+            var collector = new LambdaClosureUsageCollector(lambda);
+            collector.Visit(lambda.Body);
+
+            var captured = collector.CapturedParameters;
+            for (var i = 0; i < captured.Count; ++i)
+                _tree.LambdaClosureParameterUsages.Add(new LambdaClosureParameterUsage(
+                    checked((short)lambdaNodeIndex),
+                    checked((short)GetId(ref _parameterIds, captured[i]))));
+        }
+
+        private sealed class LambdaClosureUsageCollector : System.Linq.Expressions.ExpressionVisitor
+        {
+            private readonly System.Linq.Expressions.LambdaExpression _lambda;
+            private readonly List<SysParameterExpression> _scopedParameters = new();
+            private readonly HashSet<SysParameterExpression> _scopedParameterSet = new(ReferenceParameterComparer.Instance);
+            private readonly HashSet<SysParameterExpression> _capturedParameterSet = new(ReferenceParameterComparer.Instance);
+
+            public readonly List<SysParameterExpression> CapturedParameters = new();
+
+            public LambdaClosureUsageCollector(System.Linq.Expressions.LambdaExpression lambda)
+            {
+                _lambda = lambda;
+                for (var i = 0; i < lambda.Parameters.Count; ++i)
+                {
+                    var parameter = lambda.Parameters[i];
+                    _scopedParameters.Add(parameter);
+                    _scopedParameterSet.Add(parameter);
+                }
+            }
+
+            protected override Expression VisitLambda<T>(System.Linq.Expressions.Expression<T> node) =>
+                // Intentionally skip nested lambdas: each lambda closure map is collected independently
+                // when that lambda node is visited by the parent Builder traversal.
+                ReferenceEquals(node, _lambda) ? base.VisitLambda(node) : node;
+
+            protected override Expression VisitParameter(SysParameterExpression node)
+            {
+                if (!_scopedParameterSet.Contains(node) && _capturedParameterSet.Add(node))
+                    CapturedParameters.Add(node);
+                return node;
+            }
+
+            protected override Expression VisitBlock(System.Linq.Expressions.BlockExpression node)
+            {
+                var initialScopeCount = _scopedParameters.Count;
+                for (var i = 0; i < node.Variables.Count; ++i)
+                {
+                    var variable = node.Variables[i];
+                    _scopedParameters.Add(variable);
+                    _scopedParameterSet.Add(variable);
+                }
+                var result = base.VisitBlock(node);
+                for (var i = _scopedParameters.Count - 1; i >= initialScopeCount; --i)
+                    _scopedParameterSet.Remove(_scopedParameters[i]);
+                _scopedParameters.RemoveRange(initialScopeCount, _scopedParameters.Count - initialScopeCount);
+                return result;
+            }
+
+            protected override SysCatchBlock VisitCatchBlock(SysCatchBlock node)
+            {
+                var initialScopeCount = _scopedParameters.Count;
+                if (node.Variable != null)
+                {
+                    _scopedParameters.Add(node.Variable);
+                    _scopedParameterSet.Add(node.Variable);
+                }
+                var result = base.VisitCatchBlock(node);
+                for (var i = _scopedParameters.Count - 1; i >= initialScopeCount; --i)
+                    _scopedParameterSet.Remove(_scopedParameters[i]);
+                _scopedParameters.RemoveRange(initialScopeCount, _scopedParameters.Count - initialScopeCount);
+                return result;
+            }
+
+            private sealed class ReferenceParameterComparer : IEqualityComparer<SysParameterExpression>
+            {
+                public static readonly ReferenceParameterComparer Instance = new();
+
+                public bool Equals(SysParameterExpression x, SysParameterExpression y) => ReferenceEquals(x, y);
+
+                public int GetHashCode(SysParameterExpression obj) => RuntimeHelpers.GetHashCode(obj);
+            }
+        }
+
         private int AddConstant(System.Linq.Expressions.ConstantExpression constant) =>
             _tree.Constant(constant.Value, constant.Type);
 
diff --git a/test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs b/test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs
index edf89600..44808035 100644
--- a/test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs
+++ b/test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs
@@ -50,7 +50,9 @@ public int Run()
             Flat_blocks_with_variables_tracked_from_expression_conversion();
             Flat_goto_and_label_nodes_tracked_from_expression_conversion();
             Flat_try_catch_nodes_tracked_from_expression_conversion();
-            return 33;
+            Flat_lambda_closure_parameter_usages_tracked_for_nested_lambda_from_expression_conversion();
+            Flat_lambda_closure_parameter_usages_excludes_nested_lambda_locals();
+            return 35;
         }
 
 
@@ -931,5 +933,69 @@ public void Flat_try_catch_nodes_tracked_from_expression_conversion()
 
             Asserts.AreEqual(1, fe.TryCatchNodes.Count);
         }
+
+        public void Flat_lambda_closure_parameter_usages_tracked_for_nested_lambda_from_expression_conversion()
+        {
+            var p = SysExpr.Parameter(typeof(int), "p");
+            var sysLambda = SysExpr.Lambda<Func<int, Func<int>>>(
+                SysExpr.Lambda<Func<int>>(p),
+                p);
+
+            var fe = sysLambda.ToFlatExpression();
+
+            Asserts.AreEqual(1, fe.LambdaClosureParameterUsages.Count);
+
+            var nestedLambdaIndex = GetSingleNestedLambdaIndex(ref fe);
+            var pId = GetParameterIdByName(ref fe, "p");
+
+            var usage = fe.LambdaClosureParameterUsages[0];
+            Asserts.AreEqual(nestedLambdaIndex, usage.LambdaNodeIndex);
+            Asserts.AreEqual(pId, usage.ParameterId);
+        }
+
+        public void Flat_lambda_closure_parameter_usages_excludes_nested_lambda_locals()
+        {
+            var p = SysExpr.Parameter(typeof(int), "p");
+            var local = SysExpr.Variable(typeof(int), "local");
+            var nested = SysExpr.Lambda<Func<int>>(
+                SysExpr.Block(new[] { local }, SysExpr.Assign(local, p), local));
+            var sysLambda = SysExpr.Lambda<Func<int, Func<int>>>(nested, p);
+
+            var fe = sysLambda.ToFlatExpression();
+
+            Asserts.AreEqual(1, fe.LambdaClosureParameterUsages.Count);
+
+            var pId = GetParameterIdByName(ref fe, "p");
+            var localId = GetParameterIdByName(ref fe, "local");
+            var usage = fe.LambdaClosureParameterUsages[0];
+            Asserts.IsFalse(pId == localId);
+            Asserts.AreEqual(pId, usage.ParameterId);
+        }
+
+        private static short GetSingleNestedLambdaIndex(ref ExprTree fe)
+        {
+            var nestedLambdaIndex = -1;
+            for (var i = 0; i < fe.LambdaNodes.Count; ++i)
+            {
+                var lambdaIndex = fe.LambdaNodes[i];
+                if (lambdaIndex == fe.RootIndex)
+                    continue;
+                if (nestedLambdaIndex != -1)
+                    throw new InvalidOperationException("Expected a single nested lambda.");
+                nestedLambdaIndex = lambdaIndex;
+            }
+            return checked((short)nestedLambdaIndex);
+        }
+
+        private static short GetParameterIdByName(ref ExprTree fe, string name)
+        {
+            for (var i = 0; i < fe.Nodes.Count; ++i)
+            {
+                ref var node = ref fe.Nodes[i];
+                if (node.NodeType == ExpressionType.Parameter && string.Equals((string)node.Obj, name, StringComparison.Ordinal))
+                    return checked((short)node.ChildIdx);
+            }
+            throw new InvalidOperationException($"Parameter node '{name}' was not found.");
+        }
     }
 }