The voltammetric response of Zn in NaOH solutions containing different Na2S concentrations showed that for potentials located between the beginning of Zn anodic oxidation and Zn passivation, ZnS formation can be observed as a result of an initial competitive adsorption between OH- and sulphide ions for Zn sites. The anodic layer on Zn in alkaline solution changes from a ZnS layer at high Na2S concentration to a ZnO/Zn(OH)2 layer in Na2S-free solution. A complex layer, probably involving a defective ZnS layer with ZnO domains is produced within a certain OH-/sulphide ion concentration ratio range in the solution. The stability of these layers depends on the applied potential.