The surface modification of carbon supported Pt fuel cell catalysts by attachment of a sulfonated silane is reported. Attachment of the silane to the carbon surface before Pt deposition yields higher performances than attachment after Pt deposition. Optimum performances for both types of modified catalyst were reached at a 10% Nafion loading in the fuel cell catalyst layer, and were significantly better than that for an untreated standard catalyst at 10% Nafion loading. However, the optimum performance of the untreated catalyst, at 30% Nafion loading, was slightly better than that of the best treated catalyst. These results are explained by the fact that both optimized catalyst layers contained approximately the same concentration of sulfonate groups.