The double-layer impedance of Pt electrodes with and without adsorbed CO was investigated in 0.5 M H2SO4 by a fast Fourier transform method. The complex impedance plot for a smooth bare Pt electrode was a straight line at an angle of 4-degrees-6-degrees to the vertical. This was explained in terms of atomic-scale surface unevenness, considering the distributed pore model of a flat surface coupled with the solution resistance. However, when the Pt electrode was modified by adsorbed carbon monoxide. the observed impedance behavior was purely capacitive at frequencies above 10 kHz. This was explained in terms of the hydrophobic nature of the adsorbed CO.