In this work a systematic study of the behaviour of a Nafion 117 membrane under conditions of elevated temperatures (up to 150 degrees C) and pressure (up to 0.7 MPa) was carried out. Attention focused primarily on the ionic conductivity of the membrane in the proton form with exposure to the conditions under study for up to 800 h. The ion-exchange capacity, morphology, FTIR and NMR spectra of the membrane were determined to explain the decline in conductivity observed over time. The techniques used did not reveal any chemical degradation of the membrane polymer. The morphological changes to the membrane connected with excessive expansion of the internal structure of the polymer are assumed to be the reason for the phenomenon observed. Finally, to confirm the conclusions derived, the membrane behaviour in a laboratory-scale water electrolysis cell was studied under operating conditions corresponding to its prior characterization. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.