This paper introduces a novel method to improve CO2 permeability and CO2/light gas permselectivity by adding poly(ethylene glycol) (PEG) into the PEG-silica organic-inorganic hybrid matrix. The matrix is cross-linked by in-situ formation of silica nanopartides with diameters ranging from 1 to 5 nm. These particles disperse homogenously in the organic phase according to the morphology studied by scanning TEM. Much small particles were evolved after blending PEG into the matrix due to more fraction of bifunctional and trifunctional silicon containing groups. Four PEGs with different molecular weights (M-w = 400, 1000, 1500, and 2000 g/mol) are added into the matrix prior to the matrix formation. Permeability coefficients of three pure gases (H-2, N-2, and CO2) and one mixed gas (CO2/N-2) are measured to explore the effects of PEG content and molecular weight on the gas transport properties. The membrane containing 60 wt % of 1000 g/mol PEG could achieve an ultrahigh CO2 permeability of 845 barrer with CO2/H-2 and CO2/N-2 permselectivity around 10 and 40, respectively. Fundamental studies on the effect of temperature on gas diffusivity and solubility are also carried out. Molecular weights of PEGs have been proved to play a crucial role in determining the permeability and diffusivity.