In ideal biofilters, organic pollutants are essentially converted into mineral end products, while biomass is produced at low rates. Nevertheless, in real cases, excess biofilm develops within the porous medium, and thus causes clogging of fixed-bed bioreactors. This paper presents a characterization of biofilters clogging. Two identical, laboratory-scale biofilters have been operated in parallel, for a 2-month period, for the removal of toluene vapors. The total air flow rate was set at 1 m(3) h(-1) and the toluene concentration at 1.9 g m(-3). The periodic supply of incremental nitrogen concentrations led to the progressive and cumulative formation of biomass, and the consequential increasing pressure drop. This nutrient addition allowed to control biofilter performance, and to achieve elimination capacity values around 90-95 g m(-3) h(-1), in both units, for nitrogen flow rates near 0.4 g N day(-1). Relationships were developed to evaluate the biofilm thickness (similar to510 mum) and to correlate the pressure drop with the bed porosity, according to an equation inspired by Ergun. Further, four methods of clogging control have been tested. In the first time, two mechanical methods have been employed: bed stirring and bed washing, which permit removal of part of the excess biofilm, without affecting the performance (removal efficiency greater than or equal to80%). Secondly, two chemical methods, based on the nutrient (nitrogen) control, have also been evaluated. However, they did not lead to significant control of pressure drop and their corresponding performance was not satisfactory (removal efficiency less than or equal to60%). (C) 2003 Elsevier Science B.V.