We have investigated the hydrogen absorption/desorption hysteresis by means of in situ powder X-ray diffraction (XRD) and solid-state H-2 NMR to clarity the location of hydrogen, surface or body, and its chemical form, molecular, atomic, or as hydride. The present results point out that strongly trapped hydrogen atoms exist inside the Pd nanoparticles due to a strong Pd-H bond formation and are stabilized in the lattice of Pd nanoparticles, compared to bulk Pd.