A novel type of bioreactor for waste gas treatment has been designed. The reactor contains a microporous hydrophobic membrane to create a large interface between the waste gas and the aqueous phase. To test the new reactor, propene was chosen because of its high air/ water partition coefficient, which causes a low water concentration and hampers its removal from air. Propene transfer from air to a suspension of propene-utilizing Xanthobacter Py2 cells in the membrane bioreactor proved to be controlled by mass transfer in the liquid phase. The resistance of the membrane was negligible. Simulated propene transfer rates agreed well with the experimental data. A stable biofilm of Xanthobacter Py2 developed on the membrane during prolonged operation. The propene flux into the biofilm was 1 x 10(-6) mol m(-2) s(-1) at a propene concentration of 9.3 x 10(-2) mol m(-3) in the gas phase.