Using a rotating Pt electrode, the simultaneous electro-oxidation of methanol and electroreduction or O2 was studied in O2 saturated 0.5M H2SO4 solutions containing dissolved methanol. The two processes occurred with no apparent interaction above 0.8 V vs. a reversible H-2 electrode. At potentials below 0.8 V the net oxidation/reduction current is smaller (i.e., less cathodic) than would be expected if the reactions occurred with no interaction. The evidence suggests that the effect is due to partial poisoning of the O2 reduction process by adsorbed methanol. At sufficiently low potentials (approximately 0.4 V), O2 electroreduction reaches the same diffusion-limited whether or not methanol is added. The mechanism of O2 electroreduction appears to remain unchanged in the presence of methanol.