Sterile peat used as biofilter medium, was amended with four different concentrations of a nutrient solution, inoculated with a toluene-utilizing consortium. Samples were incubated over 120 days in closed environments microcosms with water vapor and toluene saturation to emulate the conditions prevailing in biofilters. An improvement on toluene consumption rates, associated with microbial growth was observed with high nutrient concentrations. However, at longer periods (over 60 days) toluene consumption decreased to cell maintenance levels. The CO2 Production rate was associated with toluene degradation. scanning electron microscopy (SEM) micrographs revealed a non-homogenous biofilm distribution. Higher microbial cell density, salt crystals and extracellular polymer formation was observed with higher nutrient concentrations while scarce colonization and cell morphology changes were observed with lower nutrient concentrations. The results of this work corroborate the importance of nutrient concentration on biofilter performance during start-up and long-term operation. (C) 2002 Elsevier Science Ltd. All rights reserved.