An activated sludge aeration basin (20 cm i.d., 140 cm height) equipped with either a coarse air diffuser (a plastic pipe perforated with 56 orifices of 2 mm in diameter) or a fine diffuser (porous plastic type with 100-micrometer pores) was utilized to treat an air-borne hydrophobic VOC (toluene, 700-800 mg/m3). The purposes of this study were to test the influences of both MLSS and diffuser type on the VOC removal efficiency. Results show that higher MLSS (mixed liquor suspended solids) such as 10,000-40,000 mg/L in the mixed liquor did not enhance greatly the transfer and removal of the introduced toluene. Instead, activated sludge basins with a normal MLSS (e.g., 2000-4000 mg/L) in the mixed liquor and an efficient gas diffusion system with volumetric VOC transfer coefficient of around 10-15 1/h can be used for the removal of hydrophobic VOCs from the introduced gas. For achieving a removal of over 95% of the introduced toluene or similar hydrophobic VOCs, commercial air diffusers for aerobic biological wastewater treatment basins can be used with a submerged liquid depth of over 0.40 m over the diffusers and an aeration intensity (air flow rate/basin cross-sectional area) of lower than 5.0 m3/m2.h.