The deposition of Cu onto stepped Au(111) surfaces from sulfuric acid solutions has been studied in the underpotential and overpotential range in order to elucidate the role of surface defects in deposition kinetics. Au(111) surfaces of different step densities have been prepared by cutting four single crystals between 0.7 and 4.5 degrees off their [111] axis. Au(111) surfaces roughened by oxidation-reduction cycles were also included in the study for comparison. Current transients revealed a marked influence of the step density on the nucleation rates, both for structural transitions within the Cu adlayer at underpotentials and for bulk deposition. Variation of the step density allowed for an assignment of voltammetric features to the underpotential deposition of Cu at steps or terraces. For bulk Cu deposition the critical free energy of nucleation and the size of the critical Cu nucleus were determined.