The permeability coefficients of O-2, N-2, and CO2 gases at 25 degrees C were examined for composite membranes that were prepared by filling poly(ethylene oxide)(PEO) with different molecular weights into a porous membrane. The permeability coefficients of O-2, N-2, and CO2 were 2 x 10(-10) - 4 x 10(-10), 5 x 10(-11) - 9.5 x 10(-11), and 6 x 10(-10) -1 x 10(-9) (cm(3) STPcm/cm(2) s cmHg), respectively. The higher permeability coefficients of CO2 are explained in terms of high solubility of CO2 in filled PEG. The permeability coefficient of CO2 was affected by the degree of crystallinity of PEO in the composite. On the other hand, there was little effect of crystallinity on O-2 and N-2 permeability coefficients. Some probable relationships between selectivities of O-2 to N-2 and CO2 to N-2 and the degree of crystallinity of PEO were observed. The CO2 gas permeability coefficients of the composite membrane for PEO50000 (M-w = 5 x 10(4)) showed a marked change due to melting or crystallization of PEG.