The passive behaviour of zinc in an alkaline pH 13.0 solution under dark and illumination conditions has been studied using dc polarization and ac impedance techniques. Illumination with polychromatic light in the wavelength region 200–900 nm caused significant dissolution of anodically formed passive layers on zinc. This was evident from increased anodic currents, a ten-fold decrease in the charge transfer resistance and an increase in the capacitance of the passive layer. The photo-induced dissolution effect was only slightly evident under open-circuit conditions, where a precipitated or adsorbed layer forms on the electrode. These results are explained in terms of the n-type semi-conducting properties of ZnO.