Recalcitrant volatile organic compounds with low biodegradabilities pose challenges for biofiltration technologies. In this study, the effects and mechanism of adding ozone on the performance of a biofilter were investigated. A biofilter treating chlorobenzene was set up and operated continuously for 265 days under different inlet ozone concentrations. Results showed that ozone below 120 mg m(-3) could notably enhance the biofilter performance. The average chlorobenzene removal efficiency increased from 40 to 70% and then to 90% while the inlet ozone concentration rose from 0 to 40 mg m(-3) and 120 mg m(-3). Reducing ozone concentration resulted in a decrease in removal efficiency from 90 to 40%. Further analysis indicated that the thickness and extra-cellular polymer substance content of the biofilm were remarkably reduced while inlet ozone concentration was gradually increased. Meanwhile, the specific surface areas of the filter bed were found to increase from 784 to 820 and 880 m(2) m(-3). A respiratory quinone profile showed that the dominant quinone shifted from ubiquinone-8 to menaquinone-9(H(2)) after ozone was added. This indicated that some Gram-positive bacteria with thick cell wall became the dominant species under ozone compression.