A detailed analysis of a biotrickling filter treating trichloroethene (TCE) vapors anaerobically is presented and discussed. The biotrickling filter relies on mixed cultures containing bacteria from the genus Dehalococcoides that reductively dechlorinate TCE to ethene. After about 200 days of steady operation, as biomass in the packed bed increased, a partial loss in treatment performance was observed which prompted the present investigations. Analysis of TCE and of its degradation metabolites in the gas phase and in the trickling liquid combined with the calculation of global effectiveness factors revealed that significant mass transfer limitations existed. Depending on the conditions, either the gas film or the liquid film limited the removal of TCE. These findings were confirmed by the determination of gas and liquid films mass transfer coefficients. In all cases, removal of TCE was greater without trickling of liquid. The most plausible reason for the onset of mass transfer limitations was the decrease in the specific interfacial area brought by important biomass growth over time. Overall, this study illustrates how complex kinetic and transport limitations can vary with the operating conditions in biotrickling filters. (C) 2009 Elsevier Ltd. All rights reserved.