Biotechnological techniques, including biofilters and biotrickling filters are increasingly used to treat air polluted with VOCs (Volatile Organic Compounds). In this work, the start-up, the effect of the gaseous ammonia addition on the toluene removal rate, and the problems of the heat accumulation on the performance of a laboratory scale biofilter were studied. The packing material was sterilized peat enriched with a mineral medium and inoculated with an adapted consortium (two yeast and five bacteria). Start-up showed a short adaptation period and an increased toluene elimination capacity (EC) up to a maximum of 190 g/m(3)/h. This was related to increased CO, outlet concentration and temperature gradients between the packed bed and the inlet (Tm-Tin). These events were associated with the growth of the microbial population. The biofilter EC decreased thereafter, to attain a steady state of 8 g/m(3)/h. At this point, gaseous ammonia was added. EC increased up to 80 g/m(3)/h, with simultaneous increases on the CO, concentration and (Tm-Tin). Two weeks after the ammonia addition, the new steady state was 30 g/m(3)/h. In a second ammonia addition, the maximum EC attained was 40 g/m(3)/h, and the biofilter was in steady state at 25 g/m(3)/h. Carbon, heat, and water balances were made through 88 d of biofilter operation. Emitted CO2 was about 44.5% of the theoretical value relative to the total toluene oxidation, but accumulated carbon was found as biomass, easily biodegradable material, and carbonates. Heat and water balances showed strong variations depending on EC. For 88 d the total metabolic heat was -181.2 x 10(3) Kcal/m(3), and water evaporation was found to be 56.5 kg/m(3). Evidence of nitrogen limitation, drying, and heterogeneities were found in this study. (C) 1998 John Wiley & Sons, Inc.