The objective of this paper is to define and validate a mathematical model that describes the physical and biological processes occurring in a trickle-bed air biofilter for waste gas treatment. The model assumes a two phase system, quasi-steady state and one limiting substrate. Experimental data from the biodegradation of toluene in a pilot system with four packed bed reactors, are used to test the validity of the model. The unknown biofilter variables are estimated using a non-linear parameter estimation technique. Using these parameter values, simulations were carried out for different operational conditions, and the model predicitions were compared to experimental data.