The simultaneous extraction of Co(II) and Mg(II) from nickel sulfate solutions has been carried out using the organophosphonic extractant Ionquest 801 diluted in Exxsol D-80. Statistical design and analysis of experiments were used in order to determine the main effects and interactions of the solvent extraction parameters, which were the extraction pH at equilibrium, the temperature, the extractant concentration and the organic/aqueous phase ratio. A statistically designed experiment was also carried out to study the stripping of the Ionquest 801 organic phase loaded with cobalt and magnesium by sulfuric acid solution. The number of stages required for both extraction and stripping processes of cobalt and magnesium was evaluated. The results of continuous counter-current mini-plant tests demonstrated the simultaneous recovery of cobalt and magnesium from nickel sulfate solution. (c) 2005 Society of Chemical Industry.