The aim of this work is to use small angle neutron scattering (SANS) to probe the vapour sorption of the ionomeric component of novel proton exchange membranes as a function of humidity and temperature. Experiments will utilise the faster data acquisition capabilities of Sans2d, along with its extended q-range, to obtain improved understanding of the in-situ evolution of ionomeric structure of membranes during de-hydration and re-hydration under operationally relevant conditions. The reversibility, or otherwise, of the membranes following repeated humidity cycling will also be assessed. This study will focus on new chemically stabilised and highly proton conductive perfluorosulfonic acid ionomers which enable superior fuel cell performance and durability over the much studied Nafion ionomer.