On the basis of the model proposed by De Visscher and Van Cleemput for methane oxidation in landfill cover soils, simulation models for biofiltration of toluene-contaminated air were performed for biofilters with or without substrate inhibition and operated at constant or varying airflow rates. The toluene oxidizing activity (V(max)) expressions of the derived models clearly indicate that V(max) is not a constant along a biofilter height. The equations derived when V(max) is set to zero can be used to predict the minimum concentration requirement for a viable biofiltration process. The simulation results show that the model of De Visscher and Van Cleemput, modified to incorporate Haldane kinetics, provides a good description of the biofiltration performance for toluene removal biofilters. Predictions of the minimum concentration that can be treated indicate that conventional biofilters are not suitable for indoor air pollutant removal.