A series of amine-containing polyurethanes and poly(urethane-urea)s based on 4,4＇-diphenylmethane diisocyanate and either poly(ethylene glycol) of molecular weights 400 or 600 were prepared as gas separation membranes. The amine functional groups of N-methyldiethanolamine (MDEA) and/or tetraethylenepentamine (TEPA) were introduced into the hard segment as a chain extender. The gas transport data of He, H-2, O-2, N-2, CH4 and CO2 in these polymer membranes were determined by using the Barrer＇s high-vacuum technique and the time-lag method. The restriction of chain mobility has been shown by the formation of hydrogen bonding in the soft segment and hard-segment domains, resulting in the increase in the density, glass transition temperature of soft segments (T-gs). The separation mechanism of various gas pairs used in industrial processes is also discussed. Effect of pressure on permeability of the gases above and below T-gs was studied. It was found that the gas permeability increased or decreased with upstream pressure above T-gs, and should be described by a modified free-volume model. On the other hand, the condensable CO2 exhibits a minimum permeability at a certain upstream pressure below T-gs. The permeability of He and H-2 were pressure independent above and below the T-gs.