Abstract: When deriving tests for checking functional and nonfunctional requirements for components of control systems, the notion of separablity becomes very important that is used for distinguishing the fault-free component from a faulty one. In order to do this, proper separating sequences are utilized. Such sequences are well studied for complete and deterministic Finite State Machines but components of control systems can be only partially described and their behavior can be nondeterministic. In this paper, we consider the formal model of Input/Output automata, introduce the notion of a separating sequence for two such automata and propose an approach for deriving such a separating sequence.