Mixtures of airborne toluene and o-xylene, two relatively recalcitrant volatile organic compounds (VOCs), were treated effectively using integrated UV-biofiltration. The set-up consisted of a biofilter receiving UV-pretreated stream and a reference biofilter receiving no pretreatment. Experimental conditions included UV fluences of 6 and 12mJcm(-2) as well as air flow rates of 6.3 and 9.4 L min(-1), corresponding to biofilter empty bed retention times (EBRTs) of 45 and 30 s, respectively. The inlet concentration of organics (toluene and o-xylene) ranged between 70 and 650 mg(carbon) m(-3). The UV-biofilter consistently provided removal efficiencies of greater than 95% over the range of toluene and o-xylene inlet concentrations. Also, the coupled UV-biofiltration system provided up to 60% additional contaminant removal compared to the sum of that offered by UV and reference biofilter, demonstrating the synergistic effect of UV on biofilter performance. The UV photooxidation partially oxidized a fraction of toluene and o-xylene into water soluble and more biodegradable intermediates, such as acetaldehyde and formaldehyde, which were readily removed in the downstream biofilter. These intermediates along with up to 20 ppmv ozone, formed through the photolysis of oxygen by 185 nm UV, contributed to the enhanced degradation of parent VOCs in the biofilter as well as the absence of any inhibitory effects of the VOCs on one another. Also, the presence of ozone helped control the growth of excess biofilm in the UV-coupled biofilter. While the standalone biofilter showed significant pressure drop increase (of up to 14 mm H2O m(-1) of the bed) over the course of experiment, the UV-coupled biofilter maintained a relatively low pressure drop of less than 3 mm H2O m(-1) of the bed. (c) 2006 Elsevier B. V. All rights reserved.