The aim of this work is to study how the characteristics of the polymer used to manufacture gas separation membranes influence its permeability and selectivity. It has been shown that the gas diffusivity decreases with the kinetic diameter of the gas except for CO2, probably due to its high condensability. While solubility increases with the gas condensation temperature and clearly with the glass transition temperature of the polymer for each gas. The permeabilities Of CO2, CH4, O-2, N-2 increase for increasing glass transition temperatures. Nevertheless only the selectivity Of CO2 versus the other gases increases significantly when polymers with high glass transition are used. The Robeson limit in a selectivity-versus-permeability plot is approached for CO2/CH4 when T-g, increases. This distance to the Robeson limit, for this pair of gases, results to decrease for increasing T-g For the case of the O-2/N-2 selectivity remains approximately constant with an appreciable increase in permeability for polymers with increasing Tg. Permeability increases due to the corresponcling increase in fractional free volume, FFV, that appears for increasing glass transition temperatures, T-g, This correlation of FFV with T-g has been confirmed by obtaining FFV by different methods. (C) 2007 Wiley Periodicals, Inc.