Parallel reactions during methanol electro-oxidation at smooth Pt electrodes lead to the formation of considerable amounts of HCHO and HCOOH together with CO2. In the present paper, the yields of different products at 0.6 V have been measured, using smooth polycrystalline and Pt(111) electrodes. Analysis of solution samples after ca. 15 min of electrolysis was performed via HPLC. The well-known effect of the base electrolyte on the rate of methanol oxidation is observed: lower currents are obtained in 0.5 M H2SO4 than in 0.1 M HClO4 solutions. The reason for this effect is rationalized by comparing the distribution of products in both electrolytes. This indicates that strong adsorption of sulfate hinders methanol oxidation by inhibiting the dissociative steps where the adsorbed CO intermediate is formed. The reactions forming the other soluble products, HCHO and HCOOH are not affected by the presence of H2SO4. It is suggested that the latter processes occur by an Eley-Rideal mechanism, while the reaction forming CO2 follows a Langmuir-Hinshelwood mechanism. (C) 2004 Elsevier B.V. All rights reserved.