Poly(ethylene oxide)-segmented polyurethanes (PEO-PUs) and polyamides (PEO-PAs) were prepared, and their morphology and CO2/N-2 separation properties were investigated in comparison with those of PEG-segmented polyimides (PEO-PIs). The contents of the hard and soft segments in the soft and hard domains, W-HS and W-SH, respectively, were estimated from glass-transition temperatures with the Fox equation. The phase separation of the PEO domains depended on the kind of hard-segment polymer; that is, W-HS was in the order PU > PA much greater than PI for a PEO block length (n) of 45-52. The larger W-HS of PUs and PAs was due to hydrogen bonding between the oxygen of PEO and the NH group of urethane or amide. The CO2/N-2 separation properties depended on the kind of hard-segment polymer. Compared with PEG-PIs, PEO-PUs and PEG-PA had much smaller CO, permeabilities because of much smaller CO2 diffusion coefficients and somewhat smaller CO, solubilities. PEO-PUs also had a somewhat smaller permselectivity because of a smaller solubility selectivity. This was due to the larger W-HS of PEO-PUs and PEO-PAs, that is, a greater contamination of PEO domains with hard urethane and amide units. For PEO-PIs, with a decrease in n to 23 and 9, W-HS became large and CO2 permeability decreased significantly, but the permselectivity was still at a high level of more than 50 at 35 degrees C.