Composting of leaves and alfalfa (i.e. active compost) was used for thebiofiltration of toluene-contaminated air in a 6-L biofilter (initial bedheight: 180 mm). During the thermophilic phase (45 to 55 °C), toluenebiodegradation rates reached 110 gtoluene.m-3.h-1 at an inlet concentration ofabout 5 g.m-3.h-1 and a gas residence time of 90 seconds. Thehighest rates were obtained late in the thermophilic phase suggesting amicrobial adaptation was occurring. Biodegradation rates decreased rapidly(50% in 48h) in the cooling stage. Under mesophilic conditions, themaximum biodegradation rates that could be obtained by increasing the inlettoluene concentration were near 89 gtoluene.m-3.h-1 which issimilar to that reported in the literature for mature compost biofilters. Novolatile by-product was detected by gas chromatography. Mineralization of14C-toluene and benzene showed that they were completelydegraded into CO2 and H2O under boththermophilic and mesophilic conditions. Bacteria isolated from latemesophilic stage had the capacity to degrade all BTEX compounds but were notable to transform chlorinated compounds. No organisms were isolated whichcould use toluene as their sole source of carbon and energy at 50 °C.Active compost biofiltration should be an excellent process for thetreatment of gaseous BTEX by biofiltration. This is the first report ofthermophilic biofiltration of toluene.