The removal of toluene by biofiltration has been investigated. The first objective was to study the impact of the filter bed pellet size on the biodegradation performance. Three different pellet sizes (5.0, 10 and 20 mm diameter) were evaluated with an air flowrate of 1.0 m(3) h(-1), toluene inlet concentrations between 1.8 and 3.9 g m(-3) and with variable nitrogen concentrations in the irrigation solutions (0.0-7.0 g of N l(-1)). It was shown that pellet size and specific surface area were major limiting factors for the biodegradation process. Thus, for the optimal N-concentration (2.0-3.0 g of N l(-1)), the maximum elimination capacities, decreasing with pellet size, but increasing with specific surface area, were achieved: 180 g m(-3) h(-1) (5.0 mm, 590 m(-1)), 90 g m(-3) h(-1) (10 mm, 280 m(-1)), and 45 g m(-3) h(-1) (20 mm, 120 m(-1)). The second objective was the determination of the adsorption isotherm (22degreesC) for toluene on the wet filter bed. Toluene inlet concentrations between 0.0 and 3.2 g m(-3) were introduced in a bench-scale biofilter. The experimental adsorption isotherm was modeled by either a Langmuir model (K-1 = 344.8 mug g(-1) of moist bed, K-2 = 16.7 g m(-3)) or a Freundlich approach (k(f) = 19.4 mug g(-1) of moist bed, n = 0.92).