In this study the behaviour and structure of a Nafion (R) 117 membrane exposed to liquid water at temperatures and pressures of up to 150 degrees C and 700 kPa, respectively, were investigated. The results clearly showed that, as the temperature of the environment approaches or exceeds the glass transition temperature of the membrane, it undergoes extensive swelling. This is connected with several changes in the membrane structure which have a predominantly detrimental influence on the functional properties of the membrane, such as ionic conductivity and ionic exchange capacity. The extent and rate of the changes increase with both rising temperature and pressure. While the changes at 150 degrees C and 700 kPa take place in the order of tens of minutes, at 110 degrees C and 500 kPa several hundreds of hours are required. The changes are enabled by amorphisation of pseudo-crystalline domains which are responsible for keeping the membrane structure intact. The changes in the internal structure were followed using small-angle X-ray scattering and visualised by current-sensing atomic force microscopy. (C) 2018 Elsevier Ltd. All rights reserved.