The influence of thiourea on the initial stages of Cu deposition on an Au(111) electrode has been studied by potential-step experiments and by in situ scanning tunneling microscopy. It is shown that adsorbed thiourea prevents the formation of a uniform Cu monolayer at underpotentials. The resulting island structure of the underpotential deposited layer provides many nucleation sites on the gold terraces and hence affects the nucleation habit of bulk Cu markedly. From an analysis of the current transients the critical nucleation parameters, size, and free enthalpy of formation of the critical nucleus, have been derived. The possible role of thiourea in the observed changes of the nucleation parameters and the deposition rates with the additive concentration is discussed.