Special reaction systems were proposed to investigate the mechanism of the mass transfer process in the membrane absorption process. The mass transfer resistance of this process was calculated by using hydrophobic polypropylene (PP) hollow fiber membrane (HFM); and a 2-order partial differential model, including reaction, was developed to describe the SO2 and CO2 absorption process using HIM. Throughout the experimental analyzing and model results, for either SO2/air/NaOH system or CO2/air-NaOH system, different phenomena in these two systems were discussed. Since the reaction mechanisms of both the SO2-NaOH system and the CO2-NaOH system were different, the SO2 and CO2 mass transfer in liquid side were different. The relation between the chemical reaction mechanism and mass transfer, instead of the ionized effect, was the reason for deviation in SO2 absorption process. The influence of liquid phase velocity and the concentration of absorbent on the mass transfer process was also discussed.