The lattice dynamics of α-AlH 3 and α-AlD 3 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 AlH 3 heat capacity derived from the INS data agrees with the available experimental dependence at 50-150 K.