The main purpose of this work is to establish the upper line boundary of permeability versus selectivity plots for gas separation of CO2, N-2 and CH4 binary mixtures, by means of diffusion in polyimide membranes. Previous works reported the upper bound line for CO2/CH4 mixtures, by choosing the best visual fit with respect to available data, which was theoretically justified by Freeman [1,2]. In the present case the theory was applied for molecular pairs formed between CO2, N-2 and CH4. The log-log plots of permeability (P-A/B) versus selectivity (alpha(A/B)) for polyimide polymeric materials lead to a linear upper bound with a slope fitting well the molecular kinetic diameter difference between the constituents of the gas pair. The present work contributes to establish the position predicted by theory of the upper bound line for gas separation Of CO2/CH4, N-2/CH4 and CO2N2 binary mixtures, by diffusion in polyimide membranes. The use of polyimide membranes gives a means for enriching the natural gas mixtures and their caloric potential, which is usually depleted by the presence Of CO2 and N-2 in the gas mixtures resulting from gas production, and secondary oil recovery operations. (c) 2007 Elsevier B.V. All rights reserved.