While the permeance of membranes is typically constant for ideal gases, this behavior changes for non ideal gases and vapors. Due to swelling of the polymer, its simultaneous softening (plasticization) as well as clustering effects of the penetrant molecules, permeability becomes activity dependent. While most experimental investigations focus on studying the influence of teed activity, permeate activity is often neglected. By means of constant-volume variable-pressure measurements we show the importance of both teed and permeate vapor activity for a system of pure water vapor and a variety of membrane materials. We find that a change in activity can enhance or reduce membrane permeability. Either teed or permeate activity was identified to determine the overall mass transfer. Additionally the membrane support revealed a significant impact on overall mass transport properties. This is surprising since concentration polarization effects known from mixed-gas measurements can be excluded. After more than half a decade of water transport studies, well known systems still render transport properties difficult to comprehend. While we succeed to categorize types of transport behavior, fundamental questions on the origin of the transport complexity remain a challenge for future research, identification of engineering relationships for further module and process development is established however successfully. (C) 2015 Elsevier B.V. All rights reserved.