Glycerol-based liquid membranes immobilized in the pores of hydrophilic microporous hollow fibers have been studied for selective separation of CO2 from a mixed gas (CO2, N-2) feed having low concentrations of CO2 characteristic of gases encountered in space walk and space cabin atmosphere. The immobilized liquid membranes (ILMs) investigated consist of sodium carbonate-glycerol or glycine-Na-glycerol solution. Based on the performances of such liquid membranes in flat hydrophilic porous substrates [Chen et al., Ind. Eng. Chem. Res. 38 (1999) 3489; Chen et al., Ind. Eng. Chem. Res. 39 (2000) 2447], hollow fiber-based ILMs were studied at selected CO2 partial pressure differentials (Deltap(CO2), range 0.36-0.50 cmHg), relative humidities (RH range 45-100%), as well as carrier concentrations. The sodium carbonate concentration was primarily 1.0 mol/dm(3): the glycine-Na concentration was 3.0 mol/dm(3). The sweep gas was always dry helium and it flowed on the shell side. Very high CO2/N-2 selectivities were observed with porous polysulfone microfiltration membranes as substrate. As in the case of flat film-based ILMs (see references above), feed side RH is an important factor determining the ILM performances. Generally, lower permeances and greater CO2/N-2 selectivity values were observed at lower feed stream RHs. When the feed side average RH = 60%, p(CO2.f) = 0.005 arm and glycine-Na concentration was 3.0 M, the CO2/N-2 Separation factor observed was over 5000. Prolonged runs lasting for 300 h showed that the hollow fiber-based ILM permeation performances were stable.