When hydrogen sulphide gas was introduced into a single reactor, in which the cells were mixed with iron solution in the absence of the immobilization particles, the cells were damaged by the high concentration of H2S and the H2S removal rate was rapidly decreased. Therefore, a hybrid reactor was developed, by combining a chemical reduction reactor and a biological oxidation reactor, to remove the toxic effect of H2S on the cells and to improve the H2S removal rate. The microbial cells were immobilized on the surface of curdlan particles, in order to enhance the Fe(II) oxidation rate through repeated fed-batch operation. As a result, the iron oxidation rate was four times faster than that obtained with the free cells. Iron solution, oxidized in an oxidation reactor by Thiobacillus ferrooxidans, was fed into the iron reduction reactor, and the reduced iron solution was recycled into the iron oxidation reactor. The X-ray diffractometer (XRD) data indicated that iron was precipitated along with elemental sulphur at the high concentration of H2S, resulting in the iron oxidation rate being decreased with increasing reaction time. (c) 2004 Elsevier Ltd. All rights reserved.