Mixed matrix membranes (MMMs) for carbon dioxide (CO2) separation composed of a commercial aromatic polyimide as a host matrix and carbon nanoparticles as a filler phase have been prepared by a casting method. The solubility of CO2 in the MMM could be predicted within error from the pure component isotherms, suggesting that the sorption of polymer to the nanoparticles did not significantly reduce the availability of sites for CO2 to adsorb. The CO2 permeability increased with filler loading without a reduction in the gas selectivity, reflecting the increase in fractional free volume provided by the carbon nanoparticles. This was reflected in significant changes in the CO2 diffusivity. However, contrary to prior work, the use of the carbon nanoparticles was unable to reduce the effects of plasticisation by either CO2 or water. Changes in the permeability of both water vapor and carbon dioxide occurred with time, particularly for relative humidities beyond 60%. The lack of plasticization resistance may reflect the use of sub-micron nanoparticles rather than larger ones. (C) 2017 Elsevier B.V. All rights reserved.