The change of mass transfer resistance with time has been examined for membrane-based carbon dioxide absorption in water. A commercial polypropylene hollow fiber membrane module was used with gas flow through the lumen side and liquid cross-flow on the shell side. Experiments were carried out for a prolonged period of time to evaluate absorption performance. Absorption flux decreased significantly with time due to a developing resistance to mass transfer. However, the initial flux value was restored after membrane drying, indicating that the additional resistance was reversible. A theoretical model was developed to analyze and predict flux deterioration with time in terms of partial resistances in series. The resistance against pure gas mass transfer was considered to be the sum of the liquid phase resistance and the resistance of the membrane; the latter arises from membrane pore filling by gas or liquid or both. A shell side mass transfer correlation was used for the prediction of the liquid phase resistance. The experimental absorption flux decline with time was attributed to gradual partial pore filling by liquid, and it was used to estimate the magnitude of the membrane resistance and its temporal variation. Model predictions, in good agreement with experimental results, permitted the estimation of liquid penetration into the membrane matrix. Although this was relatively low (∼13%), the resulting resistance of the liquid-filled pores accounted for over 98% of the membrane resistance and for 20-50% of the total resistance to absorption.