A hollow fiber membrane bioreactor was investigated for control of air emissions of biodegradable volatile organic compounds (VOCs). In the membrane bioreactor, gases containing VOCs pass through the lumen of microporous hydrophobic hollow fiber membranes. Soluble compounds diffuse through the membrane pores and partition into a VOC degrading biofilm. The hollow fiber membranes serve as a support for the microbial population and provide a large surface area for VOC and oxygen mass transfer. Experiments were performed to investigate the effects of toluene loading rate, gas residence time, and liquid phase turbulence on toluene removal in a laboratory-scale membrane bioreactor. Initial acclimation of the microbial culture to toluene occurred over a period of nine days, after which a 70% removal efficiency was achieved at an inlet toluene concentration of 200 ppm and a gas residence time of 1.8 s (elimination capacity of 20 g m(-3) min(-1)). At higher toluene loading rates, a maximum elimination capacity of 42 g m(-3) min(-1) was observed. In the absence of a biofilm (abiotic operation), mass transfer rates were found to increase with increasing liquid recirculation rates. Abiotic mass transfer coefficients could be estimated using a correlation of dimensionless parameters developed for heat transfer. Liquid phase recirculation rate had no effect on toluene removal when the biofilm was present, however. Three models of the reactor were created: a numeric model, a first-order fiat sheet model, and a zero-order flat sheet model. Only the numeric model fit the data well, although removal predicted as a function of gas residence time disagreed slightly with that observed. A modification in the model to account for membrane phase resistance resulted in an underprediction of removal. Sensitivity analysis of the numeric model indicated that removal was a strong function of the liquid phase biomass density and biofilm diffusion coefficient, with diffusion rates below 10(-9) m(2) s(-1) resulting in decreased removal rates.