In this paper, we propose the compositional test method (C-method), which exploits the structure of component-based communication sys tems. The C-method first tests each component separately for output and/or transfer faults, using one of the traditional test methods, then checks for composability, and finally tests the composite system for composition faults. To check for composability and to derive the test suite for the detection of com position faults, it is not required to construct the global state machine. In stead, all information is derived from the component state machines, which avoids a potential state explosion and lengthy test cases. Furthermore, the test suite checks for composition faults only. This substantially reduces the size of the test suite and thus the overall test effort.