LaNi 5 is a very important intermetallic compound for hydrogen storage applications. Its homogeneity range at 12001C has been investigated by single-crystal X-ray diffraction, electron probe microanalysis, and mass density measurements. At the nickel-rich end the hexagonal structure (CaCu 5 type) extends to the composition LaNi 5.45 . The excess of nickel is accommodated by random substitutions of lanthanum atoms by nickel dumbbells and by displacements of nickel atoms which are presumably correlated with the La atom substitutions. Interatomic distances suggest that the Ni atom displacements occur in directions toward vacant La atom sites, i.e., toward the center of Ni dumbbells. At the nickel-poor end the structure extends to the composition LaNi 4.88 . The excess of lanthanum is presumably accommodated by the simultaneous occurrence of lanthanum atoms and vacancies on nickel atom sites. Both models are consistent with mass density and electron probe microanalysis measurements. Hydrogenation thermodynamic properties have been measured as a function of composition. # 2002 Elsevier Science (USA)