The optic-vibrational density of states and the structural arrangement of hydrogen and/or deuterium dissolved in or compounded with the rare-earth metals (RH(D) x where 0<x≤3) have been measured by neutron vibrational spectroscopy (NVS) and neutron powder diffraction (NPD) for both the yttrium- and lanthanum-hydrogen systems. The spectra reflect the diversity in the phases encountered, from the α-phase solid solutions at low hydrogen concentrations to the superstoichiometric β-phase dihydrides above x=2 as well as the γ-phase trihydrides. The structure and dynamics of these systems are examined in detail for YH x (0<x≤3), where all three phases occur within the full concentration range, and LaH x (2≤x≤3), where the fcc metal lattice remains the stable configuration up to the trihydride stoichiometry. In particular for LaH x , NVS measurements of both pure and isotopically diluted samples indicate that the vibrational dynamics of hydrogen located in the octahedral (o) and tetrahedral (t) interstices are dramatically altered by the presence of significant x-dependent H-H interactions. NPD measurements are used for mapping out the x-dependent lattice symmetry due to the occurrence of long-range ordering of hydrogen in the o sublattice and as an aid in the interpretation of the vibrational spectra. In La hydrides prepared with H/D mixtures, both NVS and NPD confirm an isotopic enrichment of hydrogen in the o sublattice at low temperature.