This paper is devoted to study the use of experiments with Finite State Machines (FSM) for protocol passive testing optimization. We discuss how the knowledge obtained from the state identification of an implementation under test (IUT) can be utilized for its effective monitoring. Differently from active testing techniques, such identification is performed by only observing the IUT behavior. If the state identification is possible (at least partially), then this fact allows to reduce the number of properties (test purposes) to be checked at an execution point. Correspondingly, this allows to simplify and/or accelerate, i.e. optimize, the monitoring process by verifying the system behavior only at critical states against the appropriate set of properties associated to a given state. The paper discusses which ‘gedanken' experiments can be considered for this purpose and how they can be derived for various pecifications of communication protocols. The results presented in the paper are followed by an illustrative protocol example that demonstrates the efficiency of the proposed approach