A rotating diffusion cell with a porous membrane was used to study the kinetics of liquid/liquid extraction of copper ions using commercially important acidic extractants, i.e. dialkylphosphoric acids and hydroxyoximes. The reaction zone is assumed to be situated in a thin zone of varying thickness within the aqueous diffusion layer but adjacent to the aqueous/organic interface. The kinetic results were modeled by assuming a mixed control regime involving both chemical reaction and diffusion of reactants and products to and from the reaction zone as controlling steps. The "solubility" of the extractant in the aqueous phase, expressed in terms of the extractant distribution coefficient between the organic and aqueous phases, was shown to exert a considerable influence on the kinetic performance of the extractants. By the distribution coefficient being incorporated into the kinetic model, it is now possible to fully explain the behavior of the extraction rate curve with changing hydrogen ion concentration in the aqueous phase. The thickness of the reaction zone for hydroxyoximes is estimated to be of the order of 1.3 x 10(-3) mu m while for dialkylphosphoric acids it is of the order of 0.7 mu m.