The effects of successive mixing (homogenization) of packing material (peat), with or without water addition, on the removal of toluene vapors in a biofilter were studied. Over a period of 50 days, an increase in the Elimination Capacity (EC) of approximately 240% was obtained by successive mixing and water additions. After each mixing, a high EC of toluene was maintained only for a short period of 3-4 days. After this time, decreased biofilter performance was observed, probably associated with the development of dried and/or clogged zones. In the long-term experiments, an attenuation of the EC recovery was observed after successive mixing. In this case, an increase of 110% over 4 months of experiment was obtained. The global reduction of EC over time could be explained by the colonization of the biofilter by filamentous fungi which was facilitated by the mixing of the packing material. The most frequently observed fungi were identified as Scedosporium sp, and Cladosporium sp. (C) 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 68: 448-455, 2000.