Abstract Studies on absorption of CO 2 into aqueous solutions using hollow fiber membrane modules (HFMM) are reported. These studies were done in commercially available HFMM of 0.0254 m diameter and 0.20 m length, and 0.051 m diameter and 0.61 m length. The absorbents tested were water, aqueous NaOH (2 N or ∼ 8 wt%) and aqueous diethanolamine (0.5 M or 5.25 wt%). Correlations for predicting liquid and gas phase mass transfer coefficients were developed using the 0.0254 m HFMM. These correlations were found to successfully predict the mass transfer rates in the 0.051 m HFMM. Effective gas-liquid contact areas were found to be 2324 and 870 m 2 / m 3 for the 0.0254 m and 0.0510 m HFMM, respectively. These values are lower than the nominal surface area of the lumen for both contactors. The membrane mass transfer coefficients ( k m ) determined were much lower than those theoretically calculated for completely non-wetted pores for aqueous amine and NaOH absorbents indicating that the pores are partially wetted. In fact, even marginal ( < 2%) wetting of the pores can result in a membrane resistance that can be as high as 60% of the total mass transfer resistance.