The ac impedance of palladium foil and palladium-plated AISI 304 stainless steel in deaerated 1 moll −1 H 2SO 4 was determined at the open-circuit potential of each specimen. The surface of the plated electrode was rough and exhibited irregular features associated with the original surface of the substrate. Nevertheless, Nyquist plots for the palladium and palladium-plated electrodes were of a similar shape, indicating that the electrochemical behavior of the plated stainless steel resembled that of palladium. In both cases, Bode plots of log | Z| as a function of log ω exhibited a single linear region of slope −0.9 over at least four decades of frequency. It was shown that the data can be represented in terms of an equivalent circuit containing a capacitance associated with the double layer and an apparent impedance due to surface roughness.