The paper describes the influence of a varying feed composition of CO2/CH4 and CO2/N2 mixtures on the gas separation performance of integrally skinned asymmetric PES/PI hollow fibers with an effective skin thickness of 0.27 ¿m. Normally, thin membrane structures (<3 ¿m) show accelerated plasticization behavior induced by CO2 in pure gas measurements. This study shows that introducing an inert gas to the CO2 feed mixture apparently suppresses plasticization. This effect is more pronounced at higher concentrations of inert gas, supported by a continuous drop in CO2 permeance as a function of CO2 fugacity. At a concentration of 80% inert gas in the feed mixture, the CO2 permeance reduces more than 35% from its initial value, whereas the reduction is 8¿10% with 2% inert gas in the feed mixture. However, a mixed gas permeation model predicts for all experimentally used gas compositions similar decreases in CO2 permeance. Plasticization effects seem to be counterbalanced by competitive sorption. This effect becomes larger with increasing inert gas concentration. At 80% inert gas plasticization effects appear to be completely counterbalanced by competitive sorption. Besides that, for all gas compositions, the separation factor decreases with increasing feed pressure, generally assumed as an indication of plasticization. However, such a selectivity decrease is also predicted by the dual mode sorption model, which neglects effects of plasticization. More pronounced indication of plasticization effects is observed when the N2 permeance decay is followed in time after the membrane has been in contact with CO2 at elevated CO2 partial pressures. A significant enhanced N2 permeance is observed due to polymer network dilation, which decreases very slowly in time. There seems to be a subtle balance between plasticization and competitive sorption during mixed gas experiments with integrally skinned asymmetric hollow fibers, which results in the observed phenomenon.