The extraction of zinc was studied with solvents containing bis (2,4,4-trimethylpentyl)phosphinic acid and its monothio and dithio analogues. Comparative data are presented to illustrate the effect of composition (sulphur substitution) on the extraction and stripping characteristics of the three reagents. Special emphasis is given to the selective separation of zinc from sulphate solutions containing calcium. The effect of aralkyl versus alkyl groups in the molecule is also illustrated and important extractant properties such as hydrolytic stability and aqueous solubility are discussed.