The phenomenon of CO2 crossover from the anode to the cathode through a Nafion 117 membrane was confirmed by external addition and electrochemical generation of CO2 in the anode fluid and examination of the amount of CO2 in the cathode exhaust. The CO2 crossover rate for a single direct methanol fuel cell (DMFC) during operation was determined using a methanol-tolerant cathode catalyst to avoid the direct oxidation of methanol at the cathode electrode. The method of measuring CO2 crossover was suggested to be used for the correction of the experimental deviation for the measurement of methanol crossover in a DMFC. (C) 2002 The Electrochemical Society.