Introduction of a limited amount of oxygen to anaerobic bioreactors is proposed as a simple technique to lower the level of sulfide in the biogas. This paper presents the results of a bioreactor study and of batch experiments that were performed to obtain better insight into the fate of sulfur compounds and oxygen during micro-aerobic sulfide oxidation. Introduction of a low airflow (0.7-0.9 m(3) m-3 d(-1), corresponding to an O-2/S molar ratio of 8-10) to a fluidized bed reactor fed with low-sulfate vinasse was sufficient to reduce the biogas H2S-content to an undetectable level. Sulfide was initially oxidized to elemental sulfur, thiosulfate and - most probably -polysulfide. Significant sulfate production did not occur. Bioreactor sludge sampled from the reactor after three weeks' micro-aerobic operation was much faster in oxidizing sulfur than bioreactor sludge sampled during fully anaerobic reactor operation. The reaction proceeded faster with increasing O-2/sulfide ratios. (c) 2006 Elsevier Ltd. All rights reserved.