Reduction of metal ions with solid compounds can be used as a model system to verify various models for reactions between liquids and reactive solids. The reduction of dissolved ferric iron (Fe3+) to ferrous iron (Fe2+) with solid zinc sulphide (ZnS) was investigated in an isothermal, completely back-mixed batch reactor. The influence of temperature, initial concentration of ferric iron, zinc sulphide-to-ferric iron molar ratio, and sulphuric acid concentration on the reduction kinetics was investigated. Analysis of unreacted and reacted ZnS was performed by SEM-EDS and laser diffraction techniques to study the changes of the solid particles with time and to verify whether a product layer was present. The results did not reveal formation of any product layer, thus excluding the shrinking core (product layer) model. The experimental data were compared with several models proposed for solid-liquid reactions, representing first and second order kinetics, reaction rates controlled by surface reactions on spherical and cylindrical particles, as well as diffusion through a stagnant film and product layer. From the estimation results, it was concluded that a very good description is obtained with a simple model, where the reaction rate is proportional to the concentration of reduction agent, zinc sulphide, and to the concentration of ferric iron. The sulphuric acid had a crucial influence on the reduction kinetics; the rate being proportional to the concentration of the sulphuric acid in the power of 1.5. This indicates that the reaction between zinc sulphide and ferric iron goes through a state where sulphuric acid participates as an intermediate, thus the reaction takes place in the liquid phase. The model parameters were estimated by regression analysis. The results demonstrated clearly, that model discrimination per se is not enough to reveal the true mechanism of solid-liquid reactions, but a detailed study of solid particles is needed. (C) 2003 Elsevier Ltd. All rights reserved.