Crosslinked poly(ethylene glycol diacrylate) (PEGda) oligomers differing in molecular weight, and their nanocomposites prepared with up to 10 wt.-% methacrylate-functionalized fumed silica (FS) or an organically-modified nanoclay, have been examined as amorphous CO2-selective membranes. These novel materials have been characterized by dynamic rheology before and after crosslinking to ascertain the effect of incorporated FS on mechanical properties. The permeabilities of CO2, H2, N2, and O2 have been measured as functions of PEGda molecular weight, nanofiller content and temperature. In all cases, CO2 displays relatively high permeability, coupled with high CO2 selectivity, due to the specific interaction between quadrupolar CO2 and the ether linkages along the PEG backbone, and the accompanying enhancement in CO2 solubility. Variable-temperature permeation exhibits Arrhenius behavior, and the activation energy for CO2 permeation is found to be i) markedly lower than that of any of the other gases examined, and ii) independent of both PEGda molecular weight and nanofiller content.