Abstract. In the eld of reactive system programming, data ow synchronous languages like Lustre BCH+85,CHPP87 or Signal GBBG85 o er a syntax similar to block-diagrams, and can be e ciently compiled into C code, for instance. Designing a system that clearly exhibits several independent" running modes is not di cult since the mode structure can be encoded explicitly with the available data ow constructs. However the mode structure is no longer readable in the resulting program; modifying it is error prone, and it cannot be used to improve the quality of the generated code. We propose to introduce a special construct devoted to the expression of a mode structure in a reactive system. We call it mode-automaton, for it is basically an automaton whose states are labeled by data ow programs. We also propose a set of operations that allow the composition of several mode-automata parallel and hierarchic compositions taken from Argos Mar92 , and we study the properties of our model, like the...