This work presents an experimental study into the precipitation of CuS and ZnS in a semibatch-wise operated bubble column. First the applied bubble column was characterized with respect to mass transfer phenomena. The influences of ionic strength and superficial gas velocity on volumetric mass transfer coefficient and gas holdup, respectively, were determined using both CO2 and H2S gas. Increasing the ionic strength was found to increase the gas holdup and volumetric mass-transfer coefficient. Although the gas holdup with H2S was found to be higher than that with CO2 at the same ionic strength and superficial gas velocity, the measured volumetric mass transfer coefficient was for CO2 absorption. In the second part of the study the influences of the H2S gas concentration, initial metal concentration and gas pressure on the precipitation of ZnS and CuS were investigated. With increasing H2S concentration, the initial concentration or pressure of Zn ions yields a decrease in the average size of produced ZnS particles. No significant effects could be observed when producing CuS particles. This was probably a result of the surface activity of such particles, causing them to cluster and form agglomerates. The results are useful for scale-up and design of similar types of precipitator. (C) 2004 American Institute of Chemical Engineers.