The kinetics of molecular oxygen electroreduction were studied on platinum surfaces modified by tin underpotential deposition. The surface process was analysed by cyclic voltammetry in aqueous 10(-4) M tin(II)/1 M sulfuric acid in the 0.05 to 0.70 V vs RHE range. Platinum sites involving (1 1 0) planes are mainly related to tin underpotential deposition as observed in the hydrogen sorption region. Kinetic runs for oxygen reduction were performed with the rotating ring-disc electrode technique on tin-modified platinum surfaces. It was concluded that molecular oxygen reduction on tin-modified platinum takes place through bulk hydrogen peroxide and water formation. This interpretation was confirmed by calculating the reaction order with respect to oxygen. Electrochemical rate constants for oxygen reduction pathways were calculated as a function of deposition potential based on Damjanovic's reaction scheme.