The optical vibrations of hydrogen in TbNiAlH_1.4 and UNiAlH_2.0 were investigated by means of inelastic neutron scattering. The experimental data were analysed, including multiphonon neutron scattering contributions, calculated in an isotropic harmonic approximation. At least two fundamental H optical peaks were observed in TbNiAlH_1.4, and were assigned to the vibrational modes of hydrogen atoms occupying different interstitial sites in the metal sublattice. The high-energy part of the UNiAlH_2.0 spectra is characterized by strong anharmonicity, and a broad fundamental band. The latter can be accounted for by a large dispersion of phonon modes due to the strong H-H interactions, and/or different metal-hydrogen force constants, which may originate from different metal atoms surrounding the H atoms in the unit cell.