The paper describes an in situ electrochemical method for depositing platinum nanoparticles on carbon and inside Nafion, i.e., in proton exchange membrane fuel cell active layer conditions. The method is based on the association of carbon impregnation by H2PtCl6 and electrochemical deposition. It guarantees good ionic and electronic percolations, i.e., small ohmic drops, high platinum mass fractions (20 to 40% Pt/(Pt + C)), i.e., smaller active layer thickness and hence smaller diffusional limitations, and a narrow nanoparticle size distribution (2-4 nm). This provides high efficiency factors and results in high values of specific and mass activities for the oxygen reduction reaction.