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An equilibrium model is proposed for the reactive system ZnSO4/di-2-ethylhexyl phosphoric acid (D2EHPA)/diluent being considered by the European Federation of Chemical Engineering as a test system, valid over a wide concentration range of extraction and stripping conditions. The model is supported by the statistical analysis of estimated parameters. Two simple consecutive reactions are assumed to give two zinc complexes with relative proportions changing as conditions change from extraction to stripping. The model was solved by Newton''s method, and all parameters were estimated by data fitting using the direct search method of Hooke and Jeaves. The model predicts aqueous concentrations with an average relative error lower than 11% by assuming activity coefficients of 1.0. The incorporation of the activity coefficients may improve the model accuracy but cannot validate other simpler extraction mechanisms. [Copyright &y& Elsevier]