Plasticization of gas separation membranes by carbon dioxide permanently alters their performance and increases the possibility of membrane failure. This is amplified in ultra-thin composite membranes, where the active polymeric layer is less than 2 mu m. Here, the plasticization influence of CO2 is measured on ultra-thin polysulfone composite membranes for a range of active layer thicknesses, at four temperatures. The resulting permeability-pressure isotherms demonstrate plasticization occurs for all thicknesses at pressures lower than has been reported for dense membranes. These isotherms were quantitatively fitted with an expanded dual-sorption model that takes into account plasticization of the membrane. The plasticization potential Of CO2 for polysulfone was found to increase with reduced active layer thickness. Similarly, the plasticization potential of CO2 was found to decrease with temperature. These results are consistent with similar research that shows that thin films behave differently to dense membranes. (C) 2009 Elsevier B.V. All rights reserved.