Linear sweep voltammetry and cathodic polarization experiments were conducted to investigate the effects of electrolyte composition and current/overpotential on Co-Ni alloy co-deposition in sulphate solutions and its anomalous behavior whereby Co is preferentially deposited over Ni. Ni deposition was shown to be inhibited in the presence of CoSO4 in solution, whereas the rate of Co deposition was not accelerated and, in fact, completely unaffected by the presence of NiSO4. The extent of anomalous behavior was found to be affected strongly by the CoSO4 concentration, but was less pronounced when Co(II) reduction became mass transfer-controlled or the NiSO4 concentration exceeded the CoSO4 concentration by at least one order of magnitude. Analysis of the linear scans and measurement of the deposition current efficiencies indicated that the onset of reactions occurred in the following order with increasing cathodic overpotential: H+ reduction, Ni(II) reduction, Co(II) reduction and H2O reduction. Boric acid was found to have little effect on alloy composition whenever deposition occurred to any significant extent. On the other hand, the presence of boric acid was required to attain acceptably high current efficiencies over a wide range of current densities. Moreover, effective Co-Ni deposition was not possible in the absence of boric acid. (C) 2012 Elsevier Ltd. All rights reserved.