The kinetics and rate limiting sub-processes in the bacterial oxidation of zinc sulphide with Thiobacillus ferrooxidans were examined. The oxidation rate of synthetic ZnS in the presence and absence of bacteria at equal ferrous and ferric iron concentration and pH, was measured in parallel batch experiments. Bacteria oxidize Fe2+ produced in the chemical oxidation of ZnS with Fe3+. In the fermenter without bacteria hydrogen peroxide was added to regenerate ferric iron and maintain the Fe2+ concentration equal to that in the fermenter with bacteria. No significant differences in the oxidation rates of zinc sulphide were found between the slurry with and without bacteria at equal Fe2+ and Fe3+ concentrations. Consequently, an indirect mechanism (i.e. chemical oxidation of ZnS with Fe3+ to Zn2+, S0 and Fe2+, and bacterial oxidation of S0 to SO2-4 and Fe2+ to Fe3+) determines the oxidation rate of ZnS in the bacterial oxidation process. From the experiments it was shown that T. ferrooxidans prefers S0 as a substrate and the regeneration of Fe3+ can even become terminated. From the experimental data several suggestions for improved process design and experimental research are given.