Dihydrogen peroxide production on platinum particles supported on carbon inside a proton exchange membrane (Nafion(R)), that is, under the working conditions of PEMFC cathodes, is rather small at the usual oxygen reduction potentials. As on bulk platinum, a four-electron mechanism appears to be the main pathway, with particle size and carbon substrate effects on the H2O2 production. A large increase in the H2O2 contribution takes place at low potentials, that is, at the working potentials of PEMFC anodes.