The transport behavior of H2, N2, CO2 was studied for blends of an aliphatic copolyester with a copolyester-polyurethane (PU) at various temperatures from 20 to 80°C. It was found that there was an abrupt change in slope near the melting point of the copolyester in Arrhenius plot of the permeability vs. temperature. This was considered to be due to the phase changes in the blends. The relationship between the logarithm of gas permeability and blend composition were only slightly dependent on temperature owing to the low degree of crystallinity of the copolyester; however, the activation energy of gas permeation for the blends had different values below and above the melting temperature of copolyester. The Tg measurements by DSC suggested that the copolyester/PU blends are miscible. However, the gas permeation measurements revealed the blends are not molecularly homegeneous. The miscibility of the blends did not extend to the angstrom level.