A theoretical model is established for predicting the biodegradation of a low concentration volatile organic compound (VOC) in a trickling biofilter. To facilitate the analysis, the packed bed is simplified to a series of straight capillary tubes covered by the biofilm in which the liquid film flow on the surface of biofilm and the gas core flow in the center of tube. The theoretical formulas to calculate liquid film thickness in the capillary tube are obtained by simultaneously solving a set of hydrodynamic equations representing the momentum transport behaviors of the gas-liquid two-phase flow under co-current flow and counter-current flow. Subsequently, the mass transport equations are respectively established for the gas core, liquid film, and biofilm with considering the mass transport resistance in the liquid film and biofilm, the biochemical reaction in the biofilm, and the limitation of oxygen to biochemical reaction. Meanwhile, the surface area of mass transport in the capillary tube is modified by introducing the active biofilm surface area, namely the specific wetted surface area available for biofilm formation. The predicted purification efficiencies of VOC waste gas are found to be in good agreement with the experimental data for the trickling biofilters packed with empty set 8 mm, empty set 18 mm, and empty set 25 mm ceramic spheres under the gas-liquid co-current flow mode and counter-current flow mode. It has been revealed that for a fixed inlet concentration of toluene, the purification efficiency of VOC waste gas decreases with the increase in the gas and liquid flow rate, and increases with the increase in the specific area of packed materials and the height of packed bed. Additionally, it is found that there is an optimal porosity of packed bed corresponding to the maximal purification efficiency. (C) 2007 Elsevier Ltd. All rights reserved.