The separation of Co from Ni by the new extractant di (2,4,4-trimethylpentyl)phosphinic acid, H(DTMPP) or CYANEX 272, was studied by liquid-liquid extraction and by supported liquid membranes, SLM. The following extraction stoichiometrics and conditional equilibrium constants were evaluated using toluene as organic diluent and [NH4NO3[ = 0·3 M as aqueous medium Co2+ + 2 (R^fp ^ Co(HX2)2 + 2H+, Ke = 6 x 10"8 Ni2+ + 3 (H^) 5* Ni(HX2)2.(H2X2) + 2H+, Kg = 1 x 10"10 where H2X2is the dimeric form of H(DTMPP). The extraction of Co and Ni was also studied using the diluent mixture 67% decalin + 33% diisopropylbenzene, which is compatible with the polypropylene membrane supports, and a 1 M acetate buffer aqueous medium