The treatment of a gas stream containing a mixture of acetone, toluene and trichlorethylene (TCE) was tested in two biofilters packed with compost only (biofilter B1) or a mixture of pozzolan and compost (biofilter B2). The biofilter performance was evaluated at VOCs concentration of 101 mg m(-3) of acetone - 210 mg m(-3) of toluene - 89 mg m(-3) of TCE and air flow of 0.8 m(3) h(-1). Inocula were provided by the compost native microflora and microbial growth was enhanced by periodically adding an average of 150 ml of a commercial nutrient solution to each biofilter. Pollutant removal efficiencies of 100% (acetone loading of 7-13 g m(-3) h(-1)) and 80% (toluene loading of 17-21 g m(-3) h(-1)) were achieved for acetone and toluene, respectively, in biofilter B1. By comparison, these values were 100% (acetone loading of 6-11 g m(-3) h(-1)) and 90% (toluene loading 16 - 27g m(-3) h(-1) and after the complete re-moisturization of the porous medium) in biofilter B2. TCE was not eliminated in either of the biofilters tested. The compost was a good packing material which also provided a suitable microbial flora for the quick decomposition of acetone and toluene, however it was subject to clogging. Mixing this material with pozzolan solved this problem and enabled the biofilter capacity to be maintained over a long period of time.