Studies on absorption of CO2 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 similar to 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 an 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.