occam is a programming language designed to support concurrent applications, especially those implemented on networks of communicating processors. The aim of this paper is to formulate the meaning of the language constructs ofoccam by semantic definitions that are intended as a formalization of the informal descriptions usually found in programming language manuals. This is done by defining syntax directed transition systems, where the transitions associated with a phrase are a function of the transitions associated with its components. This method is by no means novel; the concepts were introduced in Ref. 2 and are applied in Ref. 3 where the operational semantics for CSP is presented. The operational semantics for a subset of Ada is defined in Ref. 5, where tasking and exception handling are modeled. In this paper an operational semantics for a subset ofoccam is presented for the first time; this includes the semantics of both static and dynamic behavior. The main novelty of this paper is the semantics of priority alternation which is defined in terms of the inability of the parallel context to communicate along certain channels.