In the present work, biofiltration of xylene vapors has been investigated on a laboratory scale biofilter packed with press mud as filter material inoculated with activated sludge from pharmaceutical industry. Four various gas flow rates, i.e. 0.03, 0.06, 0.09 and 0.12 m(3) h(-1), were tested for inlet xylene concentration ranging from 0.2 to 1.2 gm(-3). The biofilter proved to be highly efficient in the removal of xylene at a gas flow rate of 0.2 m(3) h(-1) corresponding to a gas residence time of 2.8 min. For all the tested inlet concentrations, the removal efficiency decreased for high gas flow rates. For all the tested gas flow rates, a decrease in the removal efficiency was noticed for high xylene inlet concentration. The follow-up of carbon dioxide concentration profile through the biofilter revealed that the mass ratio of carbon dioxide produced to the xylene removed was approximately 2.52, which confirms complete degradation of xylene if one considers the fraction of the consumed organic carbon used for the microbial growth. (C) 2008 Elsevier B.V. All rights reserved.