The influence of thiourea on the nucleation and growth of copper onto glassy carbon from acid sulphate solutions was studied using potentiostatic steps and cyclic voltammetry. The electrolyte was a 0.07 M CuSO4-1.8 M H2SO4 solution containing 0, 1, 10, 100 or 1000 mg l(-1) thiourea. In solutions without thiourea an instantaneous nucleation takes place at the surface. Addition of thiourea changes the initial nucleation to a progressive one. After the initial period of progressive nucleation, the nucleation process is terminated and a transition from progressive to instantaneous nucleation takes place. Thiourea increases the nucleation rate and the number density of nuclei at the surface. Since the deposit is smooth and planar at high nuclear number densities, it can be concluded that thiourea effectively promotes smoothness of the surface. The highest nuclear number densities are obtained at 10 mg l(-1) thiourea. At higher concentrations of thiourea the nucleation process is disturbed by the high amount of thiourea molecules adsorbed. A convenient diagnostic criterion for thiourea concentrations too high to produce a smooth and homogeneous surface was found in the cyclic voltammograms. An extra anodic stripping peak, possibly due to dissolution of copper-thiourea complexes adsorbed on the surface, appears at too high thiourea concentrations.