Co was electrodeposited on Au(111) from solutions of CoSO4 in 0.5 mM H2SO4. STM showed that the electrodeposition started at an underpotential of 0.26 V. In cathodic sweeps voltammograms exhibited a low cathodic peak C1 at an overpotential of 0.1 V and a large cathodic peak C2 at an overpotential above 0.2 V. From the cathodic charge it was estimated that at peak C1, the deposit was about one monolayer thick. AFM indicated the presence of flat deposits at peak C1, and of numerous islands at peak C2, demonstrating massive nucleation and growth at the latter peak. It is suggested that peak C1 may be associated with a hindrance of nucleation processes due to adsorption of chemical species, probably of hydrogen Had. During a reverse anodic sweep from C2, anodic peaks A1 and A2 were observed. Peak A1 was evidently associated with anodic oxidation of electrodeposited cobalt, whereas peak A2 at nobler potentials, probably resulted from oxidation of hydrogen, incorporated into the Co electrodeposit.