The effect of diluents on the rate of extraction of copper using di-2-ethylhexylphosphoric acid and 5-nonylsalixylaldoxime as extractants has been explored. The metal extraction rates using a number of extractant/diluent combinations have been related to some physical property of the diluents, i.e., the dipole moment, solubility parameter, dielectric constant, and interfacial surface pressure. Diluent properties that favor interaction between the extractant and the diluent result in lower metal extraction rates. The nature of interaction between the extractant and both aromatic and aliphatic diluents may be described in terms of physical parameters. The choice of the diluent is shown to affect the rate of metal extraction. Our results confirm that in order to achieve good kinetics, aliphatic diluents should be employed, but it is also known that these diluents should contain a certain percentage of aromatics in order to limit the extractant soluble losses to the aqueous effluent. Satisfactory linear correlations were observed between the extraction flux and a few diluent parameters, namely: dipole moment, solubility parameter, interfacial surface pressure, and the extractant distribution constant.