The electrochemical surface areas of Pt-black/Nafion thin-film electrodes immersed in 0.5 M H2SO4 were measured using the cyclic voltammetry (CV) technique. The decrease in the catalyst surface utilization measured at high potential scan rates was due to the incomplete H-adsorption process caused by the ohmic potential drop in the solution, which resulted in the measured electrode potential being lower than the real one during the cathodic scan. The ohmic potential drop became more appreciable when the roughness factor of an electrode or the scan rate increased, due to the increase in the current. The Nafion film coating (<= 8 mu m) had a negligible effect on the surface area measurements. The real catalyst surface area of an electrode can be accurately estimated through CV, if a scan rate that does not cause appreciable ohmic potential drops is used; the appropriate scan rate becomes lower as the roughness factor of the electrode increases.