The application of liquid surfactant membranes (LSMs) for cobalt-nickel separation in a sulphuric liquor was investigated using bis(2,4,4-trimethylpentyl) monothiophosphinic acid, Cyanex 302, as carrier. A high nickel/cobalt initial concentration ratio was adopted for the external phase, namely 63.3:1, reproducing the one related to an industrial leaching liquor from a Brazilian company. The membrane phase was composed of the carrier, the surfactant ECA 4360 and the diluent Escaid 110, while, as internal phase, a 0.5 mol/l solution of sulphuric acid containing lithium as a tracer for membrane break-up was employed. For this system, the effects of extraction pH, external phase ionic strength and operating temperature were studied. (C) 2004 Elsevier B.V. All rights reserved.