A lab-scale trickling filter for treatment of toluene-containing waste gas was investigated. The filter performance was investigated for various loads of toluene. Two levels of the gas flow were examined, 322 m d(-1) and 707 m d(-1). The gas inlet concentrations were varied in the range from 0.6 to 4.0 g m(-3). The toluene elimination increased linearly with increasing load, and at maximum load the elimination was 50 g m(-3) h(-1) (70% purification efficiency). This was in accordance with reported values for toluene removal in trickling filters. The removal was determined by the gas/liquid mass transfer and the biological degradation as well. An analytical model described the toluene removal as a half-order removal by use of two sets of parameters for the gas/liquid mass transfer and the biological degradation due to the two different gas flows. The mass transfer coefficients and the surface removal rates estimated by parameter fitting corresponded to previously observed values. The effect of the gas flow on the mass transfer coefficient and the biological removal rate may be explained by different flow patterns of the gas and the liquid phases. A characterisation of the biofilm showed an almost even biofilm growth over the filter height, which was in accordance with a constant liquid concentration throughout the column. (C) 1997 IAWQ. Published by Elsevier Science Ltd.