Oxygen reduction reaction (ORR) kinetics in acid solutions was studied by analysis of the polarization curves obtained by rotating disk electrode method for Pt(111) in HClO4 and H2SO4 solutions. The model for ORR kinetic currents assumes that the intrinsic exchange current and Tafel slope are independent of anion adsorption. The site blocking and electronic effects of adsorbed OH (in HClO4) and bisulfate (in H2SO4) were evaluated with the adsorption isotherms incorporated in the model. The best fits yielded the intrinsic Tafel slope in the range from -118 to -130 mV/dec, supporting single electron transfer in the rate-determining step with the corresponding transfer coefficients equal to 0.50 and 0.45, respectively. In addition to site blocking, a negative electronic effect on ORR kinetics was found for both OH and bisulfate with the effect of the latter being much stronger. The deviation of the apparent Tafel slope in HClO4 from its intrinsic value can be fully accounted for by the site blocking and electronic effects of adsorbed OH ions, which vary with coverage over the mixed kinetic-diffusion controlled region. For Pt nanoparticle catalysts, the apparent Tafel slope is similar to that for Pt(111) in HClO4 and the positive potential shift is mainly due to the increase in apparent exchange current as effective surface area increases.