The permeability, diffusivity, and solubility of He, Ne, Ar, and Kr were determined in poly(methyl acrylate) (PMA), in a temperature range encompassing the glass transition temperature, Tg. Activation energies for diffusion, ED, were higher above Tg than below Tg for all four penetrants in PMA and in the structural isomer of PMA, poly(vinyl acetate) (PVA). For all penetrants studied, the Tg, the magnitude of the enthalpy of mixing (ΔHm), as well as the EDwere all larger for PVA than for PMA. These differences were attributed to the stronger dipole-dipole interactions possible in PVA where the dipolar carbonyl group is separated from the chain backbone by an oxygen atom.