The presence of methanol at the cathode may cause depolarisation of the Pt cathode used for oxygen reduction in a direct oxidation methanol fuel cell. A semi-empirical model is presented to describe the simultaneous oxygen reduction and methanol oxidation processes on a carbon supported Pt gas diffusion electrode used in practical fuel cells. The model is verified against potentiostatic polarisation data measured at 80°C in sulphuric acid over a wide range of methanol concentrations using both oxygen and air as the oxidative reactant. The results confirm earlier observations that the mixed net current is formed as a sum of the anodic methanol oxidation and the cathodic oxygen reduction current. Of these, the methanol oxidation current is very little affected by the presence of oxygen, but the oxygen reduction current is suppressed by surface intermediates from the methanol oxidation process.