Volatiles with a low water solubility are difficult to remove from air by biofiltration, especially when present in low concentrations, because the driving force for mass transfer into the aqueous phase is very small. We have used a microporous hydrophobic membrane as a support for biofilms that remove the poorly soluble propene from air. Two bacterial strains were compared, and the faster growing strain, Xanthobacter Py2, was selected to study biofilm formation and reactor performance at propene concentrations in the range encountered in off-gas treatment. With a concentration of 200-350 ppm of propene in the gas phase a 20-day start-up period was required for the formation of a biofilm on the membrane. Once the biofilm had been established, continuous propene removal was tested over a range of concentrations. The amount of active biomass adapted to the amount of propene available. When the membrane bioreactor was operated with propene concentrations as low as 9-30 ppm in the gas phase, the propene removal rate was stable for several weeks, without the supply of any other source of carbon or energy to the microbial population.