A remarkable change in the shape of the cyclic voltammogram of Pt/SnO2 in 1 M sulfuric acid was observed in the potential range from 0.6 to 1.0 V vs reversible hydrogen electrode. Although no change in the shape of the hydrogen adsorption/desorption region was observed during potential cycling, both the anodic current due to the formation of platinum oxides (> ca. 0.8 V) and the cathodic current due to the reduction of platinum oxides (> 0.6 V) decreased. The current in the electric double-layer region gradually increased during potential cycling, and a new redox couple formed. The ratio of the hydrogen desorption charge to the oxide reduction charge of Pt/SnO2 linearly decreased as the logarithm of the number of platinum particles per geometric unit area of SnO2 decreased. (c) 2006 The Electrochemical Society.