The transport of CO2, CH4, N-2 and He in films of polyimide 6FDA-6FpDA was studied at different temperatures, from 25 to 45 degrees C, and Relative Humidity (RH) horn 0% to 90%. The permeability of all gases decreases with increasing RH, by factors as high as 44% for CH4 and 35% for the other gases; moreover, the relative decrease of gas permeability with respect to the dry polymer value is constant with temperature and just weakly dependent on the gas type. Pure water vapour sorption experiments were also conducted on the polymeric Films in the same RH and temperature range, showing that water solubility in 6FDA-6FpDA is almost temperature-independent. A strong correlation therefore seems to exist between gas permeability decrease and water content in the membrane, due to the fact that absorbed water molecules occupy a portion of the polymer free volume, that is no longer available for gas permeation. Based on these considerations, the humid permeation results were analysed quantitatively and modelled with a simple approach based on free volume theory. (C) 2015 Elsevier B.V. All rights reserved.