The addition of silicone oil as a non-aqueous phase (NAP) to the trickling liquid enhances the mass transfer of hydrophobic compounds from gas to liquid phase in biotrickling filters (BTFs). The present study investigated the effect of NAP volume fraction (phi; 0-20%), gas-to-liquid flow ratio (7.5, 16 and 32), and operating temperature (30 and 50 degrees C) on styrene mass transfer in a BTF under abiotic conditions using a full-factorial experimental design. The BTF was packed with a mixture of pumice and steel pall rings. The inlet styrene concentration was 0.5-1.2 g m(-3) and EBRTs were set at 1 and 2 min. The results were incorporated into a model to calculate the maximum fraction of styrene transferable from the gas to the aqueous phase (beta(s)*). The beta(s)* was determined to be 0.208-1, depending on the operating conditions. Flow ratio and phi were the most significant factors affecting styrene mass transfer (p-value = 0.0348 and p-value = 0.0230, respectively). Temperature had no significant effect on the model (p-value = 0.9322) at a 95% confidence interval. Styrene mass transfer was very sensitive to phi when phi < 10%; at phi > 10% the addition of silicone oil had a slight effect on beta(s)*. (C) 2015 Elsevier B.V. All rights reserved.