Voltammetric curves and polarization data for air/oxygen cathodes of a commercial Pt catalyst in a 0.5 M sulfuric acid solution containing methanol clearly showed the effect of forming mixed potentials, namely significant cathodic shifts of the operating voltages from the values in the absence of methanol. Organic species of carbonyl groups were detected by attenuated total reflection infrared (ATR-IR) spectroscopy implemented for monitoring the methanol crossover products at a catalyst layer of highly dispersed Pt under various simulated conditions of operating air cathodes. The results indicated that complete oxidation of methanol was more favorable at elevated temperatures and a higher oxygen partial pressure. A real time mass spectrometry identified the organic species in the cathode exhaust of an operating single-cell direct methanol fuel cell (DMFC) to be intact methanol and partial oxidation products, i.e., formaldehyde and formic acid. Depending on the DMFC operating condition, the concentration level of harmful organic gas reached several hundred parts per million as estimated from the interference-corrected mass spectrometric data. (c) 2006 The Electrochemical Society.