Current responses to an alternating potential perturbation of small amplitude for a reversible redox reaction occurring at a cylindrical electrodes of radius a are treated theoretically. Simple approximate equations are presented for the ac amplitude and the phase shift as functions of a single parameter omega a(2)/D, where omega is the angular frequency and D is the diffusion coefficient. The validity of the resulting equations is examined experimentally using cylindrical electrodes with diameters ranging from 2 to 800 mu m for the redox couple [Fe(CN)(6)](3-)/[Fe(CN)(6)](4-) in 0.1 mol dm(-3) K2SO4 aqueous solutions. Good agreement between the theoretical and experimental results is obtained.