A mathematical model has been developed that describes the precipitation of metal sulffides in an aqueous solution containing two different heavy metal ions. The solution is assumed to consist of a well-mixed bulk and a boundary layer that is contacted with hydrogen sulphide gas. The model makes use of Higbie's penetration model to calculate the transfer of gaseous hydrogen sulphide to this boundary layer. The conditions that have been used in the simulations resemble those of industrial wastewater from a zinc factory. The model predicts the rate of H2S absorption, the size distribution of the metal sulphide crystals and the selectivity of precipitation. Higher precipitation rates are predicted at higher pH values and higher H2S concentrations. In all cases considered, the rate of precipitation is fully controlled by mass transfer of H2S, higher H2S concentrations and higher specific surface areas yielding higher precipitation rates. The size of the obtained crystals is predicted to increase with H2S concentration, but to decrease with specific surface area and liquid side mass transfer. These results illustrate the importance of reactor layout and operating conditions on the process of gas-liquid precipitation. (C) 2003 Elsevier Ltd. All rights reserved.