The performance of two laboratory-scale biofilters packed with pressmud (BF1) and sugarcane bagasse (BF2) was evaluated for gas-phase xylene removal under various operating conditions. Biofilter was inoculated with a mixed culture obtained from pharmaceutical sludge. Experiments were carried out at different flow rates (0.03, 0.06, 0.09 and 0.12 m3 h-1) and inlet xylene concentrations (0.2, 0.4, 0.6 and 1.2 g m-3). A maximum removal efficiency of 99 and 95 % were achieved at an inlet concentration of 0.2 g m-3 and gas flow rate of 0.03 m3 h-1 for BF1 and BF2, respectively. The dynamic behaviour of the biofilter was tested at different process conditions through vigorous short, medium and long-term shock loads. The biofilter was found to respond apace to rapid changes in loading conditions. The stability of the biomass within the reactor was apparent from the fast response of the biofilter to recuperate and handle intermittent shutdown and restart operations.