In natural gas sweetening, gas dehydration with glycols is typically carried out upstream of membrane separation of carbon dioxide. This means that when process upsets occur, these glycols can reach the membrane unit. In this work, we study the impact of two common glycols (monoethylene glycol and triethylene glycol) on the gas transport performance of cellulose triacetate membranes. We find that the glycol absorbed into the membrane initially obstructs the permeation of CH4 and CO2, due to pore filling or antiplasticisation effects, but the permeability then increases again, indicative of polymer relaxation and a loss of crystallinity in the polymer. The smaller helium molecule is significantly less affected by the presence of the glycols, possibly because its lower solubility within glycol limits its movement through the swollen structure. However, after removing the glycols with a methanol wash, the membrane performance recovers with only a slight residual plasticisation observed. In addition, the permeation of H2S, a common contaminant within natural gas streams, was studied across a range of temperatures. At the partial pressures studied (up to 0.75 kPa), H2S had very little effect on the membrane performance even in long-term exposure for up to 300 days.