The mass transfer of water and chloroform in membrane air-stripping (MAS) was studied using a microporous polypropylene hollow fiber membrane module, with air flow on the lumen side and liquid cross-flow on the shell side. Water transport experiments showed that its mass transport decreased significantly when the membrane had been in contact with water for prolonged periods. It was hypothesized that the increased mass transfer resistance was due to water condensation in a fraction of the membrane pores. MAS of chloroform from aqueous solutions confirmed the additional mass transfer resistance with prior exposure to water. It was concluded that membrane pores were either completely air-filled or partially wetted with water during the MAS process, Existing models are able to predict the performance only for either completely air-filled or liquid-filled pores. A modified model was proposed to take into account the diffusion through partially wetted pores. The model described the data well. This hypothesis also provided a plausible explanation to the conflicting literature values of the membrane mass transfer resistance. It was found that the membrane mass transfer resistance of the partially wetted pores is two orders of magnitude higher than that of air-filled pores. The overall mass transfer coefficient was constant for initial feed chloroform concentrations ranging from 50 to 1000 ppm. Crown Copyright