Biofiltration was successfully applied to treat a mixture of hydrophilic (methanol) and hydrophobic (sc-pinene) volatile organic compounds (VOCs) from contaminated air Streams. The experimental approach involved operating two identical bench-scale biofilters with media consisting of a mixture of compost and wood chips. There was no inoculation and only microorganisms indigenous to the bed medium were used throughout the whole process. Under steady-state conditions and at 40 degrees C bed temperature, the biofilters treated up to 40-45 g/m(3) bed/h of pure alpha-pinene or 250 g/m3 bed/h of pure methanol with percentage removals of more than 90-95% and gas retention times between 20 and 60 s. For mixtures of methanol and alpha-pinene, the removal rate was measured based on the total carbon and was about 70-80 g carbon/m(3) bed/h. The presence of methanol, a hydrophilic and easily biodegradable compound, suppressed the growth of alpha-pinene degrading microbial community, thereby reducing alpha-pinene removal capacity of the biofilters. Unlike alpha-pinene, methanol was not affected by the presence of alpha-pinene and was equally removed from the air stream under different alpha-pinene loading rates. A steady-state model was also developed that considers the biofilm as an organic matrix and uses Monod kinetics plus inhibition. It was shown that for the biofiltration of alpha-pinene, the conventionally used air/water partition coefficient can not explain the high removal rates of this compound. Instead, air/biofilm partition coefficient which accounts for the lipophilic characteristics of the biofilm proved to be a proper parameter for modelling the biofiltration of hydrophobic VOCs.