Biofilters are fixed bed bioreactors with immobilized biofilm, which are used for the removal of pollutants from airstreams. Partial coating of the bed packing of a biofilter with an organic non-volatile and non-water miscible solvent before biofilm development is a possible way to improve the performance of the systems for the removal of hydrophobic organic compounds. The heavy solvent enhances the sorption capacity of the biofilter and may improve the rate of pollutant biodegradation. In this research a conceptual model for a dual liquid phase biofilter was developed. Using the model and simulation of the process under different conditions, it was shown that the addition of a solvent to the bed of a biofilter can enhance the biodegradation rate of a hydrophobic pollutant when there is efficient contact between biofilm and heavy solvent layers, and when the process is not reaction limited. The results of simulation moreover showed that the difference between the solubility of the pollutant in the solvent and in biofilm, is an important factor that determines the usefulness of heavy solvents in biofilters. When the difference is large the inclusion would be reasonable. The model was calibrated to describe the biofiltration of n-hexane in a silicone oil amended biofilter. The trends predicted by the model are in good agreement with the experimental data. (C) 2008 Elsevier B.V. All rights reserved.