Inappropriate biological treatment of dimethyl sulfoxide (DMSO) used by opto-electronics and semi-conductor industries would result in production of malodorous compounds, e.g. dimethyl sulfide, methane-thiol and hydrogen sulfide. The best sludge for DMSO biodegradation was obtained from the activated sludge of a chemical company that used to provide DMSO for the above industries. Under the optimal conditions of pH 7.0-8.5 and 30 degrees C, the highest removal efficiency in treatment of 500 mg 1(-1) of DMSO occurred at the rate of 0.078 g DMSO per gram suspended solids per day corresponding to 37 It for complete DMSO biodegradation in a shake-flask culture. However, the time needed for DMSO biodegradation could be reduced to 16 h at the rate of 0.153 g DMSO per gram suspended solids per day if a repeated-batch mode was adopted, indicating that an acclimation period is required by the DMSO degraders. The reaction time could further be shortened to less than 10 h with 95% removal of the 750 mg 1-1 DMSO at the maximum rate of 0.909 g DMSO per gram suspended solids per day using an oxygen-enriched air-lift bioreactor. No malodorous compounds, such as dimethyl sulfide, were produced revealing that the biodegradation pathway is oxidative and can solve the odor problems common in the biological wastewater treatment plant of the abovementioned industries. (C) 2007 Elsevier Ltd. All rights reserved.