This is a resubmitted proposal in a new project on water purification by membrane distillation. With this technology liquid water is excluded from passing the separating membrane, but evaporated pure water vapour is allowed to pass. The membrane should be hydrophobic, but still allow speedy passage of water vapour through pores as wide as possible, but not so wide that water can go through as a liquid. Tests of prototypes have shown that porous membranes of polytetrafluorethylene (PTFE) have these desired properties. However, the aim of our studies is to optimize the properties of the membrane for this water purification application. This proposed QENS study of water dynamics in slightly different PTFE membranes is a first step in our ambition to understand how the internal water dynamics is related to the macroscopic diffusion of water vapour through the membrane, i.e. its efficiency.