Using DD3R, silicalite and NaY zeolite membrane for gas separation is an attractive prospect for the development of a sustainable green process industry. The interest in CO2 and H-2 lies in their relatively strong and weak adsorption in zeolite, respectively, causing a peculiar behavior of membrane selectivity, which is here evaluated in a wide range of temperature (273-573 K), feed pressure (100-500 kPa) and CO2 molar fraction (0.05-0.95). As a result, CO2/H-2 selectivity of the considered zeolite is found to be high at a low temperature owing to the relatively strong CO2 adsorption with respect to the weak H-2 one. Differently, selectivity is much lower at a higher temperature because of the reduced adsorption strength. The temperature strongly affects the membrane selectivity through surface diffusion closely correlated to gas adsorption, and CO2 adsorption results responsible for the high membrane selectivity in the low temperature range.