Catalyst layer’s (CL) ionomer plays a crucial role in the operation of Proton Exchange Membrane Fuel Cell (PEMFC), not simply because it binds the catalyst nanopowder but mostly because it allows proton transport and affects both gas diffusivity and water management [1]. In the CL, the ionomer is a perfluorosulfonic acid polymer similar to that use in the membrane as electrolyte. It is in the form of a few nanometer thick film dispersed around the carbon support or as aggregates larger than 150 nm within the pores of the carbon network [2,3]. In contrast with bulk ionomer membrane or model thin films that have been extensively studied, still little is known on the distributions of ionomer, and especially of water, within the catalyst layer upon hydration [4]. However, this information is crucial to understand the operation and degradation of PEMFC.In the frame of the project FURTHER-FC [5], we perform extensive analysis of the water sorption properties, and in a more unusual way, of the structure of the catalyst layer as a function of relative humidity for different types of carbon support, using Small Angle Neutron Scattering (SANS). It allows us to estimate qualitatively the distribution of the ionomer, and to quantify the average thickness of the ionomer film coating the catalyst, as well as the location of water within the CL upon hydration. The project FURTHER-FC [2] has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under Grant Agreement No 875025. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research. Jinnouchi, R. et al. Nat. Commun. 12, (2021)Ueda, S. et al., Phys B Condens Matter 551, 309–314 (2018)Morawietz, T. et al., Fuel Cells 18, 239–250 (2018)Kusoglu et al., Chem. Rev. 117, 987−1104 (2017)https://further-fc.eu/