Coupling UV photolysis and biofiltration was evaluated as an effective treatment strategy for the enhanced degradation of hardly biodegradable aromatic volatile organic compounds (VOCs). o-Xylene, a recalcitrant and poorly water-soluble VOC, was used as a model compound and treated in two parallel treatment systems with and without UV pretreatment. Contaminated streams with flow rates of 0.186-0.384 m(3) h(-1) and inlet o-xylene concentrations of up to 0.22g m(-3) were passed through the treatment system. About 20% (between 10 and 35%) of o-xylene was converted into water-soluble intermediates during the UV photolysis stage, which partially oxidized o-xylene to more water-soluble and biodegradable byproducts. The untreated contaminant along with the byproducts of UV photolysis was then removed effectively in the biofiltration stage, with improvements of up to 100% compared with the control biofiltration process. The results suggested that combined UV photolysis-biofiltration is promising as an effective technique to eliminate hydrophobic and recalcitrant organic compounds from contaminated air steams. (c) 2005 Society of Chemical Industry.