A vapor-phase bioreactor has been developed utilizing porous metal membranes in a cylindrical design employing radial flow as opposed to traditional axial flow for the vapor stream. The system was evaluated for the biodegradation of p-xylene (p-xylene) from a water-saturated air stream by Pseudomonas putida ATCC 23973 immobilized onto sand. The biocatalyst was placed in the annular space between two cylindrical, porous stainless-steel membranes. Details of the reactor system are presented along with biological data verifying system performance. The feed flow rate and p-xylene concentration were varied between 60 and 130 cm(3)/min and 15-150 ppm, respectively. Continuous reactor operation was maintained for 80-200 h with removal efficiencies (based on p-xylene disappearance) between 80 and 95%. The effluent concentration histories were compared to determine the operating range of the bioreactor.