Results are presented concerning the separation of the mixtures of carbon dioxide, nitrogen and oxygen in membrane modules with modified polysulphone or polyimide as active layers. The feed gas was a mixture with composition corresponding to that of a stream leaving stage 1 of a hybrid adsorptivemembrane process for the removal of CO2 from dry flue gas. In gas streams containing 70 vol.% of CO2, O2 content was varied between 0 and 5 vol.%. It is found that the presence of oxygen in the feed gas lowers the purity of the product CO2 in all the modules studied, while the recovery depends on the module. In the PRISM module (Air Products) an increase in O2 feed concentration, for the maximum permeate purity, led to a rise in CO2 recovery, whereas for the UBE modules the recovery did not change.