To make a long answer short... they don't work well together. basically if you want to set a style property on a CSSStyleDeclaration and you have mx_internal declared in your class (i.e. you are overriding a flex sdk component such as a button or list) then you will get weird ambiguous reference warnings like the following: Id 1000: Ambiguous reference to setStyle The solution is simple, instead of declaring use namespace mx_internal you just have to scope each of the variables or functions you want access to. i.e. mx_internal::adobe_hidden_variable.