The extraction equilibria of Co(II) from 1·0 M KC1 medium by the commercial extractant Cyanex 302 (bis(2,4,4-trimethylpentyl)monothiophosphinic acid) dissolved in toluene has been studied. The effects of pH, extractant concentration and metal concentration on the extraction behaviour have been examined. The influence of bis(2,4,4,-trimethylpentyl)dithiophosphinic acid, one of the minor impurities of Cyanex 302 and also the major component in Cyanex 301 on the extraction of cobalt has also been considered. Experimental data have been treated graphically and numerically by means of the computer program LETAGROP-DISTR, and can be explained by assuming the extraction of the species CoR2(HR), CoR2(HR)2, CoL2and CoCIL(HL), HL being bis(2,4,4-tri-methylpentyl)dithiophosphinic acid and HR standing for bis(2,4,4-trimethylpentyl)-monothiophosphinic acid. Conditional equilibrium constants of the extraction process are also given.