The influence of water vapor on the gas permeability of a commercial polyimide, Matrimid 5218, has been extensively investigated at three different temperatures (25,35 and 45 degrees C), and with four different penetrant gases (CH4, N-2, CO2 and He), varying the relative humidity in the range 0-75%. In all tests performed, the permeability coefficient decreases as the concentration of water vapor in the membrane increases in particular, the influence of the presence of water On gas permeability is very similar for all penetrants, as the same permeability decrease is found, at a given relative humidity, despite the different thermodynamic and kinetic characteristics of the probe gases considered. As temperature is raised, the gas permeability is enhanced, as expected. On the other hand, its decrease with respect to the dry polymer values, as relative humidity increases, is not affected by temperature, and it remains substantially unaltered from 25 to 45 degrees C, suggesting that such phenomenon can be directly related to the amount of water dissolved in the membrane, which is also unaffected by temperature. Based on the experimental evidence, a simple model is proposed to describe the permeation process under humid conditions, in the framework of the free volume theory, in particular, it has been considered that absorbed water molecules influence gas permeability by occupying polymer free volume, reducing its availability to other penetrants with lower condensability. The model describes accurately the experimental data using only two adjustable parameters for the polymer-water-penetrant system, once the water solubility is estimated from sorption measurements. (C) 2014 Elsevier B.V. All rights reserved,