The simultaneous extraction and separation of Co(II) and Mg(II) from nickel sulphate solutions was carried out using the phosphinic extractant Cyanex 272. Nickel, which remained in the aqueous phase, was concentrated by Cyanex 272 and then stripped by synthetic nickel spent electrolyte in order to produce a solution suitable for nickel electrowinning. Subsequently, cobalt and magnesium separation was achieved by the thiophosphinic acid Cyanex 302. Statistical design and analysis of experiments were used, in all cases, in order to determine the main effects and interactions of the solvent extraction factors, which were the extraction pH at equilibrium, the temperature, the extractant concentration and the organic to aqueous phase ratio. A statistically designed experiment was, also, carried out in order to study stripping of the loaded, with cobalt and magnesium, Cyanex 272 organic phase by diluted sulphuric acid. Cobalt was recovered from the loaded Cyanex 302 by a synthetic cobalt spent electrolyte, in order to produce a solution suitable for cobalt electrowinning. The slope analysis method was used to determine the nature of the cobalt and nickel complexes extracted by both extractants. The number of stages required for the extraction and stripping processes was also evaluated. Finally, the results of the continuous, counter-current, mini-plant tests clearly demonstrated the feasibility of the simultaneous recovery of cobalt and magnesium from nickel sulphate solutions by Cyanex 272, as well as their separation by Cyanex 302. (C) 2004 Elsevier Ltd. All rights reserved.