The kinetics of copper underpotential deposition from sulfuric acid solutions onto ethanethiol-modified Au(III) electrodes was studied by cyclic voltammetry. We show that Cu underpotential deposition does indeed take place, although the current peak representing underpotential deposition, i.e. the deposition of the first Cu monolayer, is shifted into the overpotential range for commonly employed sweep rates. However, holding the potential in the underpotential range close to the Nernst potential for more than 1 h results in the formation of a complete Cu monolayer. We therefore conclude that, kinetically severely hindered, the Cu monolayer is formed at the Au(111)/SAM interface. Because voltammetric experiments with SAM-covered electrodes are always prone to some degree of irreproducibility, we address the influence of monolayer defects on cyclic voltammograms in a more systematic way.