Abstract The kinetic mechanisms of purifying waste gas containing toluene in low concentrations were investigated by using a biological trickling filter packed with biofilm packing-material. The type of biochemical reaction, the overall controlling factors, and the function of the biofilm were examined. The purifying process was described as a process of ‘adsorption–biochemical reaction’, with the biofilm acting as the effective adsorbing surface, and the bacterial bodies being the activating points of the biochemical reactions. An ‘Adsorption-Biofilm Theory’ and associated kinetic model were established, and the model was verified using experimental data. The correlation coefficients between calculated data and experimental data were 0.95–0.96.