Commercially available biofiltration systems have used natural bioactive materials in packed beds due to low media cost and easy availability. Detailed understanding and modeling of biofiltration systems are lacking in existing literature. Experimental studies on the isopentane treatment in air using peat- and compost-packed beds were conducted with inlet isopentane concentrations of 360 to 960 ppmv, and empty-bed gas-phase residence times of 2 to 10 min. High removal efficiences (> 90%) were achieved at low contaminant concentrations ( < 500 ppmv) and large empty-bed gas-phase residence times ( > 8 min). For both peat and compost biofilters, there was an “optimal” water content that gave the highest removal efficiency. For higher water content, mass transfer of isopentane through the liquid phase controlled the biofiltration removal efficiency. At low water content, irreversible changes in the bioactivity of peat and compost occurred, resulting in an irrecoverable loss of removal efficiency. Increases in biofilter bed temperature from 25 to 40°C improved the removal efficiency. A mathematical model incorporating the effect of water content and temperature was developed to describe the packed-bed biofilter performance. Model predictions agreed closely with experimental data.