Treatment of air streams contaminated with volatile organic compounds in a biofilter under transient and steady-state conditions of operation is described with a mathematical model. The model incorporates convection and dispersion in the gas phase, partial coverage of the solid support, interphase mass transfer between the gas and the aqueous biofilm with an equilibrium partition at the interface followed by diffusion, direct adsorption to the exposed uncovered solid adsorbent media, transfer between the biofilm and the solid support, and biological reactions in both the biofilm and the adsorbent. The model equations were solved numerically by the method of orthogonal collocation using a MATLAB computer code. The effects of pollutant dispersion in the gas phase, specific surface area available for mass transfer, thickness of the biofilm, and adsorptive capacity of the solid support on the biofilter performance were investigated in detail. The steady-state removal efficiency appears to be nearly independent of gas-phase dispersion of the pollutant in the normal industrial range of operations. Results also indicate that the biofilter performance is a strong function of specific surface area for mass transfer and biofilm thickness. Simulation results further suggest that higher adsorptive support media are capable of handling load fluctuations irrespective of the rate of reaction in the adsorbed phase.