The PdHx system is a typical model to study the fundamentals of solute intercalation and phase transformations, relevant to a broad class of applications such as evolution and electro-oxidation of molecular hydrogen in water electrolyzers and fuel cells respectively, as well as hydrogen storage, its sensing and catalysis of many hydrogenation reactions. Currently, PdHx system experiences a new life, being actively studied by various in situ structural techniques. Pd absorbs hydrogen (applying a H2 pressure or an electrochemical potential) leading to the formation of PdHx solid solution (α-phase) with slightly expanded lattice at low H content that fully transforms into a lattice-expanded β-phase non-stoichiometric hydride at higher H loading. It is well-documented that hydrogen atoms get progressively trapped in Pd under various sorption-desorption modes [1], influencing its bulk and interfacial properties. However, the intensity and progressiveness of this phenomenon remain little explored. In particular, it is of primary importance for understanding of structure–catalytic activity relationship on Pd based materials. Indeed, according to the d-band theory pioneered by Hammer and Norskov, trapping of H atoms into the Pd lattice drastically changes its electronic states, broadening its d-band, and resulting in a negative shift of the d-band center. The latter in turn leads to a change in the catalytic properties of Pd. For example, PdH0.43 NPs were shown to be more active for methanol electro-oxidation reaction compared to the pure metal Pd counterparts [2]. Schmidt et al. have also recently shown that compression of Pd topmost surface atoms leads to weaker adsorption energy of adsorbed H, enhancing the hydrogen evolution reaction rate in acidic medium [3]. In the frame of the HERMES H2020 project, [4] we shed fundamental light into the loss of H sorption capacity of 3.6 nm Pd/C nanoparticles during repeated H insertion/de-insertion cycles. By combining electrochemistry and wide-angle X-ray scattering Pd, we demonstrated that H atoms get progressively trapped into the Pd lattice. H trapping progressively expands the Pd lattice, leading to stabilization of the PdHx-β lattice. This phenomenon not only affects hydrogen evolution and oxidation reactions activities, but also manifests in various acidic (H2SO4, HClO4) and alkaline (LiOH, KOH, NaOH, CsOH) electrolytes. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement HERMES No 952184