A compost/ceramic (1:1, v/v) three section laboratory-scale biofilter inoculated with acclimated activated sludge was examined to treat high loading toluene vapors from a synthetic gas stream. The biofilter was operated continuously at different gas flow rates, 0.108-0.15 m(3) h(-1), with inlet toluene concentrations ranging 0.5-13 g m(-3). The overall performance of the biofilter was divided to seven stages according to the mode of operation (down-flow and up-flow) over a period of 102 days. Removal efficiencies ranging from 48 to 100% and elimination capacities ranging from 26 to 180 g m(-3) h(-1) were observed depending on the initial loading rates and the mode of operations. A maximum elimination capacity of 180 g m(-3) h(-1) was observed in the last period at an inlet toluene concentration of about 13 g m(-3). The results showed that changing the mode of operation (up-flow and down-flow) periodically will improve the performance of the biofilter under high inlet toluene concentration (higher than 4 g m(-3)). Results obtained in this study provide insight into the possibility of the biofilter to treat high inlet concentrations rather than low concentrations well known in the literature. (c) 2006 Elsevier B.V. All rights reserved.