There is an increasing need and desire to develop systems by combining components that are written in different languages and/or run on different kinds of machines. Success largely depends on the ability of their components to communicate and work together despite their differing backgrounds. This thesis addresses the problem of mixing two formal specification languages, SDL and LOTOS. Various approaches to mixing specification languages are examined including the SPECS approach which is presented in more detail. A unique feature of the SPECS approach is the support of multiple specification languages, including the ability to mix specifications languages within a given system design. This area of research investigates the SPECS specific mixing language ICL (Interconnection Language). The thesis looks at two formal languages, one of an asynchronous nature (SDL) and one of a synchronous nature (LOTOS), which can be combined using the Interconnection Language. Also a set of rules are given to produce this formal mixing specification from less formal descriptions. These rules use a range of informal representations and rigorous models of the required system to produce of the ICL specification. An application of these rules is presented. This research work was carried out as part of DCU’s contribution to the SPECS (Specification and Programming Environment for Communication Software) project, part of the RACE program of the EC. SPECS’s aim was to, as much as possible, automate the software development process by using formal languages. An overview of the SPECS project is presented in chapter 1 of this thesis.