Hi, I am trying to delete CustomDocumentProperties but by using following code: object[] TheArgs = new object[] { PropName, false, MsoDocProperties.msoPropertyTypeString, PropValue }; TypeOfAllCustomProps.InvokeMember("Delete", BindingFlags.Default | BindingFlags.InvokeMethod, null, AllCustomProps, TheArgs); I get this error: System.Runtime.InteropSerive.COMException (0X80020006): Unknown name. (Exception from HRESULT: ..... at Sytem.RuntimeType.InvokeDispMethod(string name, BindingFlags invokeAttr,.... somewhere i saw that you can use CustomDocumentProperties.Remove. But it is not right. thank you for your help