The language of MSC diagrams is widely used for the specification of communicating systems, the design of software and hardware for real time and reactive systems, and other industrial applications. Often it is used as an abstraction of systems specified in SDL or UML (in the form of sequence diagrams). In this paper, a novel representation of the semantics of message sequence charts is described. This formulation has been developed to enable the implementation of tools aimed at the verification of requirements for interactive systems. Our definition of the formal semantics of the language of MSC diagrams relies on the theory of interaction of agents and environments. This approach helped to simplify the definition of the semantics in comparison to other approaches based on highly sophisticated process algebras and it brought the definition of the semantics closer to possible implementations.