A membrane biofilter is used to biodegrade toluene in the gas phase. Active microorganisms are immobilized on the outside of a hollow fiber membrane cartridge and air containing toluene as the contaminant is passed through the membrane fibers. A porous polysulfone membrane containing water in the pore is used through which both toluene and oxygen are diffused to the biofilm attached to the outside surface. High conversion of toluene (84%) is achieved with a 16 second gas-phase residence time, based on total internal volume of the hollow fibers. A mathematical model is then developed to estimate toluene removal efficiencies at higher air flow rates.