Despite significant decrease in past decades, the cost of Proton Exchange Membrane Fuel Cell (PEMFC), largely due to the rare and expensive electro-catalyst made of platinum, restrains its massive deployment. Therefore, reducing the platinum content in the electrode is the keystone of intense research efforts to increase catalyst activity or utilisation. Catalyst layer structure, and especially the water and ionomer distributions, rule the active sites availability for electrochemical reactions, and thus catalyst utilisation, because of their influence on the transport of protons and oxygen. However, the rational design of more efficient electrodes faces the lack of accurate knowledge of its complex nanoporous structure. Specifically, ionomer and water distribution are very difficult to probe with conventional electron microscopy or X-Ray techniques. This work provides quantitative and new information on electrode structure, regarding ionomer and water distributions thanks to an extensive analysis of Small Angle Neutron Scattering (SANS) profiles at different relative humidity (RH) and contrast. A 2 to 3 nm thin ionomer film spreads around the Pt/C catalyst particles while a condensed water layer appears at the catalyst/ionomer interface depending on the type of carbon support.