The surface coating industry is facing increasingly stringent environmental regulations that require the control of volatile organic compounds (VOCs) and hazardous air pollutants. Biofiltration is an attractive alternative for paint spray booth applications but these facilities pose several challenges to biofiltration technology . Specifically, offgas streams from painting operations are characterized by complex VOC mixtures, frequent shutdown and restart events, high volumetric flow rates, and relatively low contaminant concentrations. The objectives of this research were to investigate the feasibility of biofiltration for paint spray booth applications and to delineate how key operating parameters and biofilter history affect the degradation of VOC mixtures and the microbial population in biofilters. The results indicate that biofilters are a feasible option for treating the emissions from paint spray booth facilities. Removal efficiencies as high as 95% or greater were achieved even under intermittent feed conditions. Biofilter performance was found to strongly depend on nitrogen supply, particularly during the start up period. Overall VOC removal was limited by the toluene and xylene components of the waste gas stream. When the inoculating culture was developed so as to maintain the degradation capacity of the culture for each VOC component of the paint mixture, a sequential feeding strategy did not provide any initial advantage with respect to VOC removal; however, the system ultimately achieved higher removals of the toluene. Biofilm samples analyzed using the DGGE technique indicate that the fungal population in the bioreactors was relatively uniform across the biofilters and stable over extended periods of operation. The DGGE banding pattern for the bacterial population, on the other hand, indicates that the bacterial community was spatially distinct and became less diverse after 200 days of operation. While this research focused on biofilters treating paint VOC mixtures, it is anticipated that many of the phenomena observed will be applicable to other bioreactor systems and contaminant mixtures.