Inner walls of microporous hollow fibers were exposed to a particle-laden gas stream for 80 h. The gas stream contained 0.25-mu m diameter particles, typical of fly ash particles found in flue gas streams, that were generated by a wet-particle technique. During the deposition period, the pressure drop across the length of the fibers increased slowly at the start of the experiment and then more rapidly later, reaching 100 in. of water in 30 h. Particle interception by fibers at the feed inlet may be the chief reason for the rise in the pressure drop. The mass transfer coefficient (MTC) for SO2 absorption into water dropped by 20% during this time. Particles coating the inside fiber walls may be the chief reason for the reduction in MTC. While the pressure drop could be reversed to its original value by the backflow of a pressurized air-jet, the MTC could not.