AC impedance studies of the highly oriented pyrolytic graphite (HOPG) basal plane were carried out in 1M NaOH at 25-degrees-C. The data were interpreted in terms of equivalent circuits generally associated with a pure intrinsic semiconductor-electrolyte interface. A five-element equivalent-circuit model was proposed and described satisfactorily the electrochemical properties of the HOPG basal plane at different potentials and frequencies. The space-charge capacitance, the surface state capacitance parameter, the resistance related to the surface states, the charge-transfer resistance, and the solution resistance were determined. The influence of hydrogen evolution on the ac impedance response of the HOPG basal plane also was studied. This reaction produces surface defects by increasing the edge-plane content. The ac technique may be used in the diagnostics of the surface changes.