The dehydriding reaction of α- Al H ₃ induced by high-energy electron irradiation was observed in situ by transmission electron microscopy and associated electron energy-loss spectroscopy (EELS). An α- Al H ₃ crystal decomposed into fine aluminum particles while retaining its initial external shape because of a thin rigid surface layer that covered the entire particle. EELS revealed that the thin surface layer was amorphous aluminum oxide, which stabilizes α- Al H ₃ particles at ambient temperature.