A general mathematical model developed for a description of pollutant degradation in a biofilm was used to evaluate the performance of a biofilter for the purification of styrene-containing gas. The biofilter contained perlite as an inert support on which a biofilm was present composed of a mixed microbial population containing the fungus Exophiala jeanselmei as a major styrene-degrading microorganism. Although styrene is a moderately hydrophobic compound, the biofilter was reaction limited at a styrene gas phase concentration of 0.1-2.4 g/m 3 . Limitation of biofilter performance by the mass transfer of styrene was only observed at styrene concentrations lower than 0.06 g/m 3 . A maximal styrene degradation rate of 62 g/(m 3 ⅐ h) was maintaind for over 1 year. At a high styrene concentration, the maximal styrene degradation rate could be increased to 91 g/(m 3 ⅐ h) by increasing the oxygen concentration in the gas from 20 to 40%. After 300 days of operation, the dry-weight biomass concentration of the filter bed was 41% (w/w), and an average biofilm thickness of 240-280 µm, but maximal up to 600 µm, was observed. Experimental results and model calculations indicated an effective biofilm thickness of about 80 µm. It is postulated that the thickness of the effective biofilm is determined by the oxygen availability in the biofilm.