The effect of surface oxidation of the carbon support of Pt catalysts with nitric acid has been investigated. Oxidation of the support before deposition of the Pt leads to a decrease in the Pt particle size, as reported by others, and enhanced performance for oxygen reduction in gas diffusion electrodes. Oxidation of the support after Pt deposition produces an even greater enhancement in performance, which we have attributed to enhanced proton conductivity in the catalyst layer. It is speculated that hydrophilic carboxylic acid groups produced by surface oxidation enhance wetting of the catalyst layer.