: Polyurethanes like other block copolymers undergo phase separation and produce a microstructure consisted of hard and soft domains. Concerning the permeability of gases through polyurethanes, soft domains act as permeable regions and hard domains act as impermeable regions making the morphology of polyurethanes an important factor affecting the permeability of gases through these materials. In this study the effect of varying the preparation conditions was investigated on the morphology and permeability of polyurethane membranes. Two batches of polyurethane based on isophoron diisocyanate/butanediol/poly(tetramethylene glycol IPDI/BDO/PTMG were synthesized, each with 30 and 50 percent hard segment content. Prepolymer bulk polymerization method was used for the synthesis of the batches. Solution casting method was used for the fabrication of the membranes and dimethylformamide (DMF) was used as solvent for dissolution of the samples. Samples were cast at 60, 80 and 100 . Permeability of ، ، and through the samples was measured using constant-volume method. Small angle X-ray scattering (SAXS) was used for studying the morphology and IR-spectroscopy (ATR-FTIR) was used for studying the hydrogen bonding in the samples. For samples with 30% hard segment, increasing the casting temperature leads to a decrease in the extent of phase separation and permeability. This phenomenon may be due to the increase in mixing of the soft and hard domains with increasing temperature. Also, increasing the casting temperature leads to a faster evaporation of the solvent and thus reduces the time available for the system to phase separate. For the samples prepared with 50% hard segment, increasing the casting temperature leads to an increase in extent of phase separation but the permeability of the studied gases go through a maximum with increasing the casting temperature from 60 to 100 . Increasing the casting temperature leads to an increase in mobility of the chains and facilitates the phase separation, on the other hand it leads to faster solvent evaporation and prevents the domains from rearrangement leading to an interpenetrated morphology with lower mobility of the soft domains. This result can be deduced from the thermal density fluctuation background obtained from SAXS experiments for the samples. Results of IR-spectroscopy for the samples prepared from both batches, do not show a significant change in hydrogen bonding with casting temperature. It can be concluded that changes in morphology do not necessarily lead to changes in hydrogen bonding within the samples. Keywords: Polyurethanes, Membranes, Gas Separation, Block Copolymers, Small Angle X-ray Scattering.