A mathematical model, taking into account the interactions between the adsorption, microkinetics, mass transfer, and gas flow patterns in biofilters, was developed to describe the transient behavior of the biofiltration process. Transient responses of biofilters as affected by the compost and by inert particles were estimated separately. In addition, time delay of the microbial growth was also considered. Simulated results were compared with previous experimental data obtained in three laboratoryscale biofilters with mixtures of chaff and compost, diatomaceous earth (DE) and compost, and granular activated carbon (GAC) and compost as the filter materials. Toluene was used as the gas pollutant in the experiments. Both the experimental and simulated results validated that the transient behavior of the biofilters was mainly influenced by the filter material properties and the operating conditions. The GAC-compost biofilter had a better transient performance than the chaff-compost and DE-compost biofilters. Moreover, the transient behaviors of the biofilters simulated by the proposed model showed good agreement with the experimental results.