The effect of the particle size of carbon-supported Pt catalysts on the electrooxidation of methanol was studied. Different methods were used to prepare Pt/C catalysts with particle sizes ranging between 1.2 and 10 nm. The possible interaction of Pt with carbon surface groups was also investigated by preparing catalysts on oxidized and non-oxidized carbon supports. The specific activity was found to decrease with decreasing particle size in the range 4.5-1.2 nm. Carbon-supported Pt particles appear to be more active than non-supported particles and the presence of an acidic group on the support slightly enhances this effect. The dependence of the activity on the particle size can be explained in terms of either its effect on the formation of an adsorbed hydroxy species or its effect on the number of methanol adsorption sites.