To provide insight into effects of UV pretreatment on a subsequent biofilters, the mechanisms of UV pretreatment on biofilter performance were further studied. Results showed that gaseous chlorobenzene UV photodegradation products resulted in a pH decrease from 6-8 to 4-7 in filter bed. Ozone produced by UV photodegradation changed the physical properties of biofilm by reducing biofilm thickness and oxidizing surface part of the extra-cellular polymer substance, which promoted the oxygen, nutrient and pollutant transfer to biofilm. The reduction of biofilm thickness further resulted in the improvement of filter bed's physical properties by providing a higher specific surface area of 880 m x m(-3) compared with the value of 784 m2 x m(-3) without UV pretreatment.