In this work, the effect of water content on biofilter performance is discussed. The inlet air humidity and media water content are two important parameters that affect biofilter performance. Using a laboratory scale biofilter unit, the effect of water content on toluene removal is studied. Water content of the packing media (peat) is varied and its effect on the removal efficiency was observed. Experimental data show that the biofilter media performed efficiently when the water content was around 50%. A good agreement between the model predictions and the experimental data was observed when experimentally obtained toluene removal efficiencies at various water contents were compared with the theoretical model predictions. Using the experimental data, correlations for biofilm surface area as a function of water content and porosity have been developed. These correlations can be useful in designing large-scale biofilter systems.