A biofiltration system using peat as solid support inoculated with Thiobacillus thioparus (ATCC 23645) was developed for treating a gaseous stream containing high concentrations of H2S. The inoculation systems allowed an efficient colonisation of the peat with T. thioparus, reaching 2.7 x 10(8) cells g(-1) of dry peat. The biofilter reached an efficiency of 100% when fed with 355 ppm H2S and an air flow of 0.030 m(3) h(-1). When increasing the inlet concentration of the acid, the removal efficiencies of H2S decreased, reaching 90 and 60%, at 0.070 and 0.030 m(3) h(-1), respectively. A maximum value for the elimination capacities of 55 g m(-3) h(-1) was obtained. Mathematical modelling based on Ottengrafs model enabled a prediction of the performance of the biofilter. (C) 2003 Elsevier Ltd. All rights reserved.