Pt/C catalysts with two different Pt crystallite sizes [as-received (20 weight percent (w/o) Pt/C powder from E-TEK) and heat-treated (900 degrees C in inert atmosphere)] were analyzed for the reduction of oxygen in half-cells using rotating disk electrodes. Measurements in HF (1.17 M) show that the specific activity of the catalysts increases with the Pt crystallite size. In contrast, measurements in H2SO4 (1.17 M) indicate that the specific activity is nearly independent of the Pt crystallite size. One of the reasons for the ciiereace in behavior observed between these acids is the difference in specific adsorption of anions on the Pt sites from H2SO4 (adsorbing) and HF (nonadsorbing) solutions. This study indicates that both the oxygen reduction reaction and anion adsorption on the Pt sites are crystallite-size sensitive in H2SO4 solution. In solid polymer fuel cells (SPFCs), the anions are anchored on the backbone of the polymer matrix, and hence only a minimal anion adsorption is expected on the Pt sites of gas diffusion electrodes. From results of the present study, it is believed that the Pt/membrane interface in SPFCs could be better mimicked by the Pt/HF interface rather than the Pt/H2SO4 interface.