The effect of Co2+ ions on zinc deposition from acidic sulfate electrolytes is investigated on the basis of steady-state polarization curves and impedance measurements. Hydrogen evolution is shown to be enhanced by adsorbed cobalt on both the zinc deposit and the aluminium substrate, and this stimulation occurs in combination with the catalytic effect of adsorbed species originating from anodic products. Hydrogen evolution involves a diffusion-controlled step and a slow adsorption process which is the rate-determining step. It also induces the redissolution of the zinc deposit observable from the potential dependencies of current, charge transfer resistance and double layer capacitance.