A mass spectrometric setup was designed which allows an on-line, and thus real-time, analysis of the products of methanol oxidation in a prototype direct methanol fuel cell (DMFC) operating at 150 to 190 degrees C. Platinum-black and platinum-ruthenium were used as anode catalysts. The methanol/water mole ratio in the anode feed and the fuel cell operating temperature were varied, and the relative product distribution was determined as a function of these parameters. For pure methanol feed, methanaldimethylacetal was found to be the main product, while an excess of water in the anode feed led to CO2 as the main product indicating almost complete oxidation of methanol. Methylformate was also detected under all conditions studied. Increase in the fuel-cell operating temperature led to an increase of the relative product distribution of CO2. Platinum black was found to be more selective toward methylformate and methanaldimethylacetal formation than platinum-ruthenium. Cycling the fuel cell anode at 1 mV/s, platinum-ruthenium showed lower onset potentials for both CO2 (similar to 0.2 vs. similar to 0.35 V vs. RHE) and methanaldimethylacetal (similar to 0.15 vs. similar to 0.3 V vs. RHE) formation than platinum.