An aqueous-organic, two-phase bioreactor system was used for the biodegradation of benzene. Through a systematic solvent selection procedure considering biological, physical, operational. environmental and economic factors, hexadecane was chosen as the most promising solvent. The solvent has infinite solubility for benzene and shows very high phase stability with aqueous media. Alcaligenes xylosoxidans Y234 capable of degrading benzene without any other co-substrate was inoculated into the aqueous phase of the two-phase partitioning bioreactor, which consisted of a 1 1 aqueous phase and 500 mi hexadecane. A feed of 7000 mg of benzene was loaded into the organic phase, which gave an initial equilibrium aqueous phase concentration of 100 mg/l. Over the course of 24 h, 63.8% of the benzene was degraded by the microorganism, and 36.2% was stripped by aeration. By installing a condenser and using a lower gas flow of pure oxygen to reduce stripping. more than 99% of a subsequent 7000 mg benzene addition was degraded by the organisms within 24 h.