A bench-scale biofilter with a 0.5-m high filter bed, inoculated with a toluene-degrading strain of Acinetobacter sp. NCIMB 9689, was used to study toluene removal from a synthetic waste air stream. Different sets of continuous tests were conducted at influent toluene concentrations ranging over 0.1-4.0 g m(-3) and at superficial gas velocities ranging over 17.8-255 m h(-1). The maximum volumetric toluene removal rate for the biofilter (242 g m(-3) h(-1)) was obtained at a superficial gas velocity of 127.5 m h(-1) (corresponding to a residence time of 28 s) and a toluene inlet concentration of 4.0 g m(-3). Under these operating conditions, toluene removal efficiency was only 0.238, which suggested that effective operation required higher residence times. Removal efficiencies higher than 0.9 were achieved at organic loads less than 113.7 g m(-3) h(-1). A macro-kinetic study, performed using concentration profiles along the bioreactor, revealed this process was limited by diffusion at organic loads less than 100 g m(-3) h(-1) and by biological reaction beyond this threshold.