Copper electrodeposition from sulfuric acid solutions onto ethanethiol-modified Au(lll) electrodes was studied by in-situ scanning tunneling microscopy and cyclic voltammetry. The ethanethiol adlayer undergoes an order-disorder transition before Cu underpotential deposition starts around +0.20 V versus SCE. Five percent of a monolayer is deposited positive of the Nernst potential at a sweep rate of 10 mV s(-1). At low overpotentials the Cu deposit exhibits a ramified monatomic high morphology, if the ethanethiol adlayer is dense. In all cases three-dimensional growth nucleating at large substrate defects is found in addition. Cyclic voltammetry revealed two characteristic de position features: firstly, a sharp cathodic peak at -0.18 V which is ascribed to the insertion of a Cu monolayer between Au and the organic adlayer and, secondly, a current loop due to Cu bulk deposition. The corresponding stripping peaks are found at 0.08 and 0.35 V, respectively.