Membrane-based solvent extraction offers many advantages over conventional solvent extraction technology including lower solvent inventories and reduced solvent losses. In this work, the extraction of cobalt with a hollow fiber contactor was studied in the context of nickel electrolyte purification. Distribution data were obtained for cobalt extraction with 2-ethylhexyl 2-ethylhexylphosphonic acid (Ionquest((R)) 801) in an aliphatic diluent, and cobalt extraction with bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex((R)) 272) in a low aromatic diluent. Mass transfer of cobalt with Ionquest((R)) 801 and Cyanex((R)) 272 as extracting reagents was carried out with a hollow fiber Liqui-Cel((R)) Extra-Flow module. The overall mass transfer coefficients were found to be much lower than those predicted assuming transport limited extraction. It was concluded that with relatively dilute cobalt concentration in the feed, reaction extraction kinetics were a major contributor to the overall resistance to mass transfer. (c) 2004 Elsevier B.V. All rights reserved.