Treatment of air pollutants in a biofilter requires that the compound be effectively transported from the gas phase to the organisms that reside in a biofilm that forms upon a packing material. Models of biofiltration generally treat the biofilm like water by using a Henry's law constant to predict mass transfer rates into the biofilm where degradation occurs and hence, predict low rates for hydrophobic compounds. However, some compounds that are virtually insoluble in water are also treated unusually well. The objective of this research is to develop and experimentally validate a model that can explain the biofiltration rates of alpha-pinene. The study involved analysing transport of alpha-pinene through artificial biofilms in a diffusion cell along with modelling of batch and continuous kinetic experiments. Our results support a novel mechanism of biofiltration whereby a biologically mediated transformation is taking place with alpha-pinene being oxidised into a more soluble compound. This model provides an explanation for relatively high removal rates of hydrophobic compounds. A simple transport and reaction model based on zero-order kinetics was developed that fit results seen in a diffusion cell using active alpha-pinene leachate immobilised in low melting point agarose. The proposed identity of this more soluble by-product, is cis-2,8-p-menthadien-1-ol, a menthadienol, a novel metabolite of alpha-pinene degradation. By extension, this model fits biofiltration data collected from Raschig ring biofilters treating alpha-pinene. The paper also discusses implications of the model for the treatment of hydrophobic pollutants. (c) 2005 Elsevier B.V. All rights reserved.