In order to understand the membrane wetting in microporous hollow fiber membrane contactors, a theoretical model was developed by simulating CO2 absorption in water under two extreme operating conditions of the non-wetted and wetted modes. The experimental studies on CO2 absorption using an aqueous DEA solution as the absorbent in a polypropylene hollow fiber membrane contactor were also conducted over a three-month period of time. Simulation results show that the CO2 absorption rate in the non-wetted mode is six times higher than those of the wetted mode of operation. The deteriorated performance in the wetted mode is mainly attributed to the mass transfer resistance imposed by the liquid in the membrane pores. The reduction of overall mass transfer coefficient may reach 20% even if the membrane pores were 5% wetted. Experimentally, the CO2 absorption in a 2 M DEA solution was found to be influenced by the gas flow rate significantly and the CO, flux was enhanced by the increase Of CO2 volume fraction in the feed stream. Moreover, in the Celgard microporous hollow fiber MiniModule(R), the CO2 flux reduced about 20% in the initial 4 days of operation and then there was no change in the performance. The membrane wetting was identified to be the main reason responsible for the performance drop, as the membrane morphology and the overall mass transfer coefficient presented corresponding changes. The retrieval of the CO2 flux to 90% of the original amount was achieved by increasing the gas phase pressure, which further validated this hypothesis. Clearly, the prevention of the membrane wetting is very critical in maintaining the high performance of CO2 absorption in the membrane contactor. (C) 2005 Elsevier B.V. All rights reserved.