The effect of urethane and urea contents on gas separation properties of the groups of polyurethane-urea membranes is studied. The membranes used in this study were prepared by polytetramethylene-glycol (PTMG) and isophorone diisocyanate (IPDI) prepolymers. The prepolymers were converted to final polymer using the designed proportion of 1,4-butane diol (BDO) and 1,4-butane diamine (BDA) as a chain extender. Five different BDO/BDA ratios were chosen in order to synthesize poly(urethane-urea)s with different urethane/urea linkage content. The prepared polymers were characterized and the permeability of gases through them was investigated. Results obtained by Fourier transform infrared spectrometer (FT-IR) and differential scanning calorimetery (DSC) indicate that by increasing the urea linkage in the polymers, the microphase separation of hard and soft segments increase. Study of the X-ray diffraction (WAXD) patterns confirmed that polyols may be arranged in small crystalline structures. Permeation measurements of polymers revealed that the permeability of gases decreases with increasing urethane content in the polymers and selectivity of gases decreases with increasing urea content. The solubility and diffusivity of gases indicate solubility domination of gas transport in these membranes. The results of permeability also show high amounts up to 128 Barrer (1 Barrer = 1 x 10(-10) [cm(3) (STP) cm/cm(2) s cm Hg]), and high selectivity for carbon dioxide with respect to nitrogen (CO(2)/N(2): 27). (C) 2010 Elsevier B.V. All rights reserved.