The state of water in cast perfluorosulphonic Nafion films has been investigated using Fourier transform infrared spectroscopy (FTIR). FTIR spectra of cast Nafion films equilibrated in different relative humidities (RH) in a non flowing vapour environment, and in 100% RH at different temperatures have been made. In contrast to previously published results on thicker membranes the FTIR experiments indicate that there is already a full proton transfer from the sulphonic acid group in Nafion when there is one water molecule per sulphonate group. (C) 2000 Elsevier Science Ltd. All rights reserved.