The electrodeposition of cobalt + copper alloy on vitreous carbon, copper and nickel electrodes in a citrate bath at pH ca. 5 has been studied for two [Co(II)]/[Cu(II)] ratios in solution. Voltammetric and stripping results on vitreous carbon show that co-deposition of the two metals takes place under these conditions. Electrodeposition leads to the formation of a solid solution of cobalt in copper, detected by only one oxidation peak that shifts from the copper to the cobalt position as the applied current density-applied potential is made more negative. Structural XRD analysis and compositional analyses, confirm the formation of a cobalt-copper solid solution of face-centred-cubic (fcc) structure. Diffractograms show peaks that appear at diffraction angles between the lines corresponding to copper and fee cobalt. The value of the fee lattice constant a of the alloy decreases linearly with the increase in the cobalt percentage in the deposits, following Vegard's law. Deposits obtained at the same charge and current density from the same bath on different substrates have similar morphology: nodular-grained deposits at low deposition current densities-applied potentials, which evolve gradually to more fine-grained and structured deposits as the cobalt percentage in the deposits is increased. (C) 2000 Elsevier Science B.V. All rights reserved.