diff --git a/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.TestCase.cs b/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.TestCase.cs
index 7d18043c5a..d233d0dd48 100644
--- a/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.TestCase.cs
+++ b/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.TestCase.cs
@@ -149,6 +149,7 @@ namespace ICSharpCode.NRefactory.TypeSystem.TestCase
 		public void MethodWithOutParameter(out int x) { x = 0; }
 		public void MethodWithParamsArray(params object[] x) {}
 		public void MethodWithOptionalParameter(int x = 4) {}
+		public void MethodWithExplicitOptionalParameter([Optional] int x) {}
 		public void MethodWithEnumOptionalParameter(StringComparison x = StringComparison.OrdinalIgnoreCase) {}
 	}
 	
diff --git a/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.cs b/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.cs
index c056c25671..b333887d13 100644
--- a/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.cs
+++ b/ICSharpCode.NRefactory.Tests/TypeSystem/TypeSystemTests.cs
@@ -605,6 +605,17 @@ namespace ICSharpCode.NRefactory.TypeSystem
 			Assert.AreEqual(0, p.Attributes.Count);
 			Assert.AreEqual(4, p.ConstantValue);
 		}
+
+		[Test]
+		public void MethodWithExplicitOptionalParameter()
+		{
+			IParameter p = GetTypeDefinition(typeof(ParameterTests)).Methods.Single(m => m.Name == "MethodWithExplicitOptionalParameter").Parameters.Single();
+			Assert.IsTrue(p.IsOptional);
+			Assert.IsFalse(p.IsRef);
+			Assert.IsFalse(p.IsOut);
+			Assert.IsFalse(p.IsParams);
+			Assert.AreEqual(1, p.Attributes.Count);
+		}
 		
 		[Test]
 		public void MethodWithEnumOptionalParameter()
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultUnresolvedParameter.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultUnresolvedParameter.cs
index 1ca05856ac..28d8feb4ce 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultUnresolvedParameter.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultUnresolvedParameter.cs
@@ -190,6 +190,11 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 			}
 			return b.ToString();
 		}
+
+		static bool IsOptionalAttribute (IType attributeType)
+		{
+			return attributeType.Name == "OptionalAttribute" && attributeType.Namespace == "System.Runtime.InteropServices";
+		}
 		
 		public IParameter CreateResolvedParameter(ITypeResolveContext context)
 		{
@@ -205,8 +210,10 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 					IsParams = this.IsParams
 				};
 			} else {
-				return new DefaultParameter(type.Resolve(context), name, region,
-				                            attributes.CreateResolvedAttributes(context), IsRef, IsOut, IsParams);
+				var resolvedAttributes = attributes.CreateResolvedAttributes (context);
+				bool isOptional = resolvedAttributes != null && resolvedAttributes.Any (a => IsOptionalAttribute (a.AttributeType));
+				return new DefaultParameter (type.Resolve (context), name, region,
+				                            resolvedAttributes, IsRef, IsOut, IsParams, isOptional);
 			}
 		}