In previous studies, the selectivity property of a porous stainless steel cylindrical tube showed different filtration rates of pure carbon dioxide (CO2) and of pure nitrogen (N-2) because of the effect of the dynamic boundary layer into the tube. In this study, a binary mixture of CO2 and N-2 is considered under three different volumetric compositions (50/50%, 60/40%, and 70/30%) to evaluate the separation property of a porous stainless steel tube (membrane effect). The pure gas permeability, mixture permeability, ideal selectivity, and separation selectivity of this tube are determined for a total mass flow rate ranging from 1 to 2.5 g.s(-1) and for an inlet pressure varying from 2 to 3.5 bar accordingly. The factors affecting the distribution of CO2 and N-2 inside the porous tube are qualified. It is found that both the ideal and separation selectivity values are close to each other for a 50/50% composition, but the difference between the values increases for different compositions because of the effects of the partial and total pressures. Both ideal and separation selectivities decrease with an increase in the CO2 concentration. The mixed gas permeabilities of both gases vary in such a manner that the selectivity (N-2/CO2) increases along the length of the tube for all the studied volumetric compositions. The concentration of CO2 in the main flow decreases along the length of the tube, whereas the concentration of N-2 increases. These findings will serve to design separation processes to be used in applications such as fuel cells or regenerative cooling on board high-speed aircraft.