Injecting air into hollow fibers and tubular membranes has been proved to be effective in order to control flux decline caused by concentration polarization and particle deposition. This paper presents a study of the characteristics of filtration with inter-fiber two-phase flow. The enhancement of flux by bubbling, the effect of the total superficial velocity and gas and liquid velocities, the effect of fiber spacing and orientation, and the concept of critical flux were investigated. A specially designed crossflow hollow fiber cell connected to a light microscope and video-camera system has been used to monitor particle deposition on the fibers. The results showed that injecting air could enhance the permeate flux and control the deposition of particles on the membrane fibers. Changes in the hydrodynamics of two-phase flow considerably affected the filtration resistance caused by reversible fouling but was ineffective for the resistance caused by irreversible fouling. The extent of deposition was mainly controlled by the flux level in the range of wall shear rates examined. A critical flux of about 10 dm(3) m(-2) h(-1) was identified through direct observation of particle deposition on fibers. This value correlated with the flux at which the irreversible fouling became negligible. These results should be significant for optimizing the operation of submerged membrane bioreactor wastewater systems in which bubbling is used as a hydrodynamic technique to improve the performance of the membrane process.