The products of methanol crossover through the acid-doped polybenzimidazole polymer electrolyte membrane (PBI PEM) to the cathode of a prototype direct methanol fuel cell (DMFC) were analyzed using multipurpose electrochemical mass spectrometry (MPEMS) coupled to the cathode exhaust gas outlet. It was found that the methanol crossing over reacts almost quantitatively to CO2 at the cathode with the platinum of the cathode acting as a heterogeneous catalyst. The cathode open-circuit potential is inversely proportional to the amount of CO2 formed. A poisoning effect on the oxygen reduction also was found. Methods for the estimation of the methanol crossover rate at operating fuel cells are suggested.