Electrochemical dissolution and passivation reactions of zinc have been studied in 1.0 M KOH solutions by electrochemical impedance spectroscopy. Equivalent circuits have been worked out by simulating the impedance data and using the results to model the dissolution and passivation reactions. A Tafel plot constructed from the charge-transfer resistances provides an exchange current of 0.11 A/cm(2) and an alpha value of 0.36 for zinc oxidation. The maximum rate of zinc oxidation is observed at about -1.30 V vs. the Hg/HgO reference electrode as judged from the charge-tranfer resistance minimum obtained from impedance measurements. A negative polarization resistance with a reverse semicircle on the Nyquist plot illustrates the transition process from an active to passive potential region at -1.10 V. At high anodic over-potentials, the zinc electrode behaved as a semiconductor electrode due to a compact ZnO passive film formed on the electrode surface.