diff --git a/src/NHibernate/Engine/ISessionImplementor.cs b/src/NHibernate/Engine/ISessionImplementor.cs
index 43f6dde729b..f32306fde2d 100644
--- a/src/NHibernate/Engine/ISessionImplementor.cs
+++ b/src/NHibernate/Engine/ISessionImplementor.cs
@@ -43,10 +43,11 @@ internal static IDisposable BeginProcess(this ISessionImplementor session)
 	/// </summary>
 	public partial interface ISessionImplementor
 	{
-		// 5.1 TODO: obsolete Initialize, it has no more usages.
 		/// <summary>
 		/// Initialize the session after its construction was complete
 		/// </summary>
+		// Since v5.1
+		[Obsolete("This method has no more usages in NHibernate and will be removed.")]
 		void Initialize();
 
 		/// <summary>
diff --git a/src/NHibernate/Impl/AbstractSessionImpl.cs b/src/NHibernate/Impl/AbstractSessionImpl.cs
index 8ac235df64e..f7d730c9bd4 100644
--- a/src/NHibernate/Impl/AbstractSessionImpl.cs
+++ b/src/NHibernate/Impl/AbstractSessionImpl.cs
@@ -55,6 +55,8 @@ protected internal AbstractSessionImpl(ISessionFactoryImplementor factory, ISess
 
 		#region ISessionImplementor Members
 
+		// Since v5.1
+		[Obsolete("This method has no more usages in NHibernate and will be removed.")]
 		public void Initialize()
 		{
 			BeginProcess()?.Dispose();