Highly permeable glassy polymeric membranes based on poly (1-trimethylsilyl-1-propyne) (PTMSP) and a polymer of intrinsic porosity (PIM-1) were investigated for water sorption, water permeability and the separation of CO2 from N-2 under humid mixed gas conditions. The water sorption isotherms for both materials followed behavior indicative of multilayer adsorption within the microvoids, with PIM-1 registering a significant water uptake at very high water activities. Analysis of the sorption isotherms using a modified dual sorption model which accounts for such multilayer effects gave Langmuir affinity constants more consistent with lighter gases than the use of the standard dual mode approach. The water permeability through PTMSP and PIM-1 was comparable over the water activities studied, and could be successfully modeled through a dual mode sorption model with a concentration dependent diffusivity. The water permeability through both membranes as a function of temperature was also measured, and found to be at a minimum at 80 degrees C for PTMSP and 70 degrees C for PIM-1. This temperature dependence is a function of reducing water solubility in both membranes with increasing temperature countered by increasing water diffusivity. The CO2 - N-2 mixed gas permeabilities through PTMSP and PIM-1 were also measured and modeled through dual mode sorption theory. Introducing water vapour further reduced both the CO2 and N-2 permeabilities. The plasticization potential of water in PTMSP was determined and indicated water swelled the membrane increasing CO2 and N-2 diffusivity, while for PIM-1 a negative potential implied that water filling of the microvoids hampered CO2 and N-2 diffusion through the membrane. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 719-728