SOFL (Structured-Object-Based-Formal Language) is a formal language and method for practical systems development. As a language, it integrates data ow diagrams, VDM-SL, and Petri nets to provide a graphical and textural formal notation for speci cation construction. Previous publications introducing SOFL are based on its informal semantics. In this paper, we provide a formal operational semantics which gives a rm foundation for consistent understanding and application of SOFL in practical systems development. Development of the semantics, and application of SOFL to practical systems, have led to some changes in the original de nitions. This formal semantics will serve as a rm foundation for building an e ective support environment for SOFL.