The purpose of this study is to evaluate an anaerobic system (ABR) cultured by chemoautotrophic bacteria for biodesulfurization of natural gas. In this study anaerobic baffled reactor with five compartments and active volume of 10 1 was used. For increasing solid retention time, reactor was packed by pumice with 0.5-2 cm diameter which active volume decreased to 91. Inoculation was performed by activated sludge from municipal sewage treatment, which was kept in anaerobic condition for 1 year. The synthetic wastewater which contains S2O32- ion was used in start-up and S-2 for running of ABR. Performance of the reactor was evaluated at three equivalent hydraulic retention times (HRT) of 50, 28, and 9 h, with the equivalent loading rate of 0.11, 0.2, and 0.62 mmol S-2 1(-1) h(-1). Sulfide was converted to elemental sulfur and sulfate, so that more than 61% of the sulfur in the end products was elemental sulfur. Throughout the experiment, biodesulfurization of sulfur waste using ABR system, showed that the process is extremely efficient, and the maximum sulfide removal rate was about 3.03 mmol S-2 1(-1) h(-1). (c) 2005 Elsevier B.V. All rights reserved.