Cobalt-nickel alloys were electrodeposited in an acid bath containing various ratios of metallic cations. The effect of the plating variables on the composition and morphology of the deposits obtained on vitreous carbon electrodes was investigated. Different proportions of the two metals can be obtained by using different deposition parameters, but at all Co(II)/Ni(II) ratios studied, preferential deposition of cobalt occurs and anomalous codeposition takes place. For a fixed solution composition, the nickel content in the deposit is enhanced by increasing the deposition potential. More homogeneous and fine-grained deposits can be obtained by increasing the cobalt(II)/nickel(II) ratio in solution and by ensuring that deposition takes place slowly. Deposits of constant composition throughout the depth of the deposit can be obtained only by stirring the solution during the deposition. In addition, the solution must be stirred in order to minimize the increase in local pH and to prevent hydroxide precipitation. An attempt was made to explain the anomalous codeposition. The results suggest the following sequence of events: first, nickel is deposited; then, cobalt(II) adsorbs onto the freshly deposited nickel and begins to be deposited. The cobalt(II) adsorption inhibits subsequent deposition of nickel, although it does not block it completely.