Electronic inter-organizational relationships are governed by contracts regulating their interaction. It is necessary to run-time monitor the contracts, as to guarantee their fulfillment. The present work shows how to obtain a run-time monitor for contracts written in CL, a formal specification language which allows to write conditional obligations, permissions and prohibitions over actions. The trace semantics of CL formalizes the notion of a trace fulfills a contract. We show how to obtain, for a given contract, an alternating B