Reflex is a process-oriented language that provides design of easy-to-maintain control software. The language has been successfully used in several safety-critical cyber-physical systems, e. g. control software for a silicon single crystal growth furnace. Now, the main goal of the Reflex language project is development a support for computer aided software engineering targeted to safety-critical application. This paper presents formal operational semantics of the Reflex language as a base for applying formal methods to verification of Reflex programs.