Assemblies and methods for measuring in-situ membrane fluid crossover are provided. One embodiment of an in-situ fuel cell membrane crossover measurement assembly as disclosed herein comprises an anode fluid supply configured to supply anode fluid to an anode side of a proton exchange membrane; a cathode fluid supply configured to supply cathode fluid to a cathode side of the proton exchange membrane; a collection chamber configured to receive an exhaust from one of the anode side and the cathode side of the proton exchange membrane; and means for detecting a crossover fluid in the exhaust. The crossover fluid is from the cathode fluid if the exhaust is collected from the anode side and the crossover fluid is from the anode fluid if the exhaust is collected from the cathode side.