Two biofilters equal in size were filled with perlite for treatment of -pinene-contaminated air. The perlite used for one of the biofilters was partially coated with silicone oil to make the surface of the particles more hydrophobic. The filters were run at 1.5, 2.5, and 5 L/min airflow rate (3.1, 1.9, and 0.9-min retention time). The filters were operated for 5.5 months. The results showed that the silicone oil-coated filter performed better at 2.5 L/min with a maximum elimination capacity of 20 g/(m³ h) in comparison with 15 g/(m³ h) for the filter without oil. The efficiency was approximately the same for both filters at 1.5 L/min (40 g/m³ h), whereas it was slightly higher for the without oil filter at 5 L/min [35 g/(m³ h)]. The flow rate was set to 2.5 L/min once more (day 151). The results showed that the elimination capacity had increased to 35 g/(m³ h) and that the efficiency of both filters was approximately the same. The difference in results between the initial run and the later run at 2.5 L/min is probably depending on that the number of microorganisms had increased during the experiment and that the oil-containing biofilter adsorbed -pinene to a higher extent than the biofilter without silicone oil during the start-up period. © 2009 American Institute of Chemical Engineers Environ Prog, 2010