The better CO2 affinity reagents, N-methyldiethanolamine (MDEA) and tetraethylenepentamine(TEPA), were used as chain extenders to prepare MDEA-extended polyurethanes, TEPA-and MDEA/TEPA(1/1 by mole)extended poly(urethane-urea)s. 4,4/-diphenylmethane diisocyanate (MDI) and poly(ethylene glycol) (PEG) of molecular weights (MW) 400 and 600 were used as hard and soft segments, respectively. The effects of the chain extenders and PEG MW on the mode of urethane moiety association and the polymer thermal transition properties were studied using differential scanning calorimetry, Fourier Transform infrared spectrophotometry (FTi.r.) and the deconvolute technique on the FTi.r. bands due to stretching vibrations of carbonyl and NH groups. The new chain extenders caused the polymers to have less ordered urethane hard segment domains and a lower T-2. They also caused an increase in the number of hard segments found in soft segment domains and a higher T-gs, compared with typical 1,4＇-butandiol-extended polyether polyurethanes. The larger MW PEG improved phase separation and exhibited a lower T-gs. The polymers were prepared as gas separation membranes for various gases such as He, H-2, O-2, N-2, CH4 and CO2. The steady state permeability (P-i), diffusivity (D-i) and solubility (S-i) of the gases permeated through the polymer membranes were determined using Barrer＇s high vacuum technique and the time lag method. Polymer membranes having PEG soft segments of MW600 showed quite promising performance (P-i, alpha(ij) = P-i/P-j) for permeation of industrially interesting gas pairs such as CO2/CH4, He/CH4, H-2/N-2, and O-2/N-2. The relation of permeation properties to the molecular properties of the penetrant gases as well as the polymer structure is discussed. The separation mechanism of the gas pairs is also discussed.