The problem of detecting multiple transistor faults (i. e. , stuck-open and stuck-on faults) in a CMOS circuit is addressed. First, it is shown that a test set that detects single stuck-open faults in any CMOS complex gate also detects all multiple stuck-open faults in it. Then conditions are derived under which a multiple fault, consisting of both stuck-open and stuck-on faults, can be guaranteed to be detectable. The results show that a large percentage of multiple transistor faults in a complex gate are, in fact, detectable. These results are still valid when the complex gate is embedded in a larger CMOS circuit.