The presence of condensable aromatic hydrocarbons in raw natural gas streams creates a significant challenge for acid gas removal through membrane separation. In this work, the impact of toluene and xylene on the gas separation performance of cellulose triacetate (CTA) membranes was studied. When operating at low CO2 partial pressures (0.75 bar), both toluene and xylene reduced the permeation of CO2 at low vapour activities, due to competitive sorption and the pore-filling or anti-plasticisation effect. Conversely, at vapour activities greater than 0.5, toluene caused membrane plasticisation, possibly coupled with a decrease in crystallinity. On the other hand, when operating at 7.5 bar CO2 pressure, plasticisation was observed at a lower vapour activity of 0.3 for both toluene and xylene. This study shows that two penetrants can influence the plasticisation behaviour in a cooperative manner that cannot be described by a simple additive model. The study also shows that the permeabilities of toluene and xylene are comparable with that of methane.