Electrolytic phase formation of cobalt onto a glassy carbon electrode (GCE) was investigated using linear sweep voltammetry and the potential step technique in aqueous 10(-2) M CoCl2 + 1 M NH4Cl at pH 4.6 and 9.5. Thermodynamic, voltammetric and spectrophotometric; analysis of the solutions showed that the predominant chemical species of Co(II) in solution were the Co(H2O)(6)(2+) ion at pH 4.6 and the Co(NH3)(5)(2+) complex at pH 9.5. Voltammetric analysis showed that the experimental equilibrium potential of the Co(NH3)(5)(2+)/Co(0) system was more negative than the Co(H2O)(6)(2+)/Co(0) couple. However the electrocrystallization overpotential for cobalt deposition on GCE from the aquo complex was higher than for the amine complex. Analysis of the current-time transients obtained at each pH, indicated that distinct mechanisms of nucleation are involved during the early stages of cobalt deposition. In the case of Co(H2O)(6)(2+) reduction, the transients were described theoretically in terms of 3D nucleation with diffusion controlled growth. For cobalt deposition from the Co(NH3)(5)(2+) species, the transients were explained by a combination of three different kinds of parallel nucleation processes, 2D progressive nucleation, 2D instantaneous nucleation and 3D progressive nucleation, each of which was limited by lattice incorporation of cobalt ad-atoms.