Specification diagrams are a novel form of graphical notation for specifying open distributed object systems. The design goal is to define notation for specifying message-passing behavior that is expressive, intuitively understandable, and that has formal semantic underpinnings. The notation generalizes informal notations such as UML’s Sequence Diagrams and broadens their applicability to later in the design cycle. \inlong Specification diagrams differ from existing actor and process algebra presentations in that they are not a programming language \emphper se; instead, like logics, they are inherently more biased toward specification. In this paper we show how it is possible to reason rigorously and modularly about specification diagrams. An Actor Theory Toolkit is used to great advantage for this purpose.