The initial stages of Co deposition onto Au(111) and Au(100) electrodes have been investigated by cyclic voltammetry and in situ scanning tunnelling microscopy (STM). Special emphasis was placed on the nucleation behaviour at surface defects such as monoatomic high steps. Depending on the crystallographic orientation of the substrate and on the electrochemical parameters, either three- or two-dimensional growth has been observed. Because Co deposition occurs at potentials, at which gold surfaces are reconstructed normally, the influence of surface reconstruction on the nucleation-and-growth behaviour is addressed briefly.