Superstoichiometric hydrides PdHx have been synthesized by the electrochemical method. The H concentrations, x = H/Pd = 1.13–1.97, have been determined directly by thermal desorption. From X-ray diffraction, the structure is found to be face-centered cubic, with the lattice parameter changing smoothly over the concentration range, x = 0.7–2.0. Surprisingly, however, the lattice parameter goes through a maximum at x ∼1.0 and decreases at higher concentrations. It is suggested from electronic and Monte Carlo calculations that these unique features of the structure and formation process should be the consequence of partial replacement of Pd atoms with H2, namely, the formation of superabundant vacancies filled with H2 molecules.