The permeability of cross-linked poly( ethylene glycol diacrylate) (XLPEGDA) to He, H-2, O-2, N-2, CO2, CH4, C2H4, C2H6, C3H6, and C3H8 was determined at temperatures ranging from - 20 to + 45 degrees C and at fugacities up to approximately 15 atm for some gases. Diffusion coefficients of CO2 and hydrocarbons were calculated as a function of temperature and local penetrant concentration. The effect of temperature and fugacity on permeability and diffusivity is satisfactorily described using either an activated diffusion model or a free volume-based model.