Аннотация:The lattice dynamics of alpha-AlH3 and alpha-AlD3 were studied by inelastic neutron scattering (INS). The hydrogen optic modes in the INS spectra were shown to form a broad structured band which exhibited an harmonic isotopic behaviour. In contrast, the low-energy part of the phonon spectra, formed mainly by the vibrations of the metal atoms, showed pronounced anharmonicity. The temperature dependence of the AlH3 heat capacity derived from the MS data agrees with the available experimental dependence at 50-150 K.