In order to elucidate the origin of the mechanical stability of ternary non-stoichiometric compounds, the influence of hydrogen gas absorption-desorption on the crystallographic lattice expansion has been investigated in single-phase LaNi¿ and LaNi¿-1Cu compounds (5.0 ¿ 6.0) by means of in situ X-ray powder diffractometry. The extent of the hydrogen plateau region where the a and ß phases coexist is strongly dependent on both the non-stoichiometric and the chemical composition of the compounds. The discrete a-ß lattice expansion rather than the total lattice expansion is found to be essential for the mechanical stability of the hydride-forming powder particles. Materials which are characterized by a large discrete lattice expansion suffer from a considerable particle size reduction, whereas materials lacking such a pronounced discrete expansion are found to be mechanically much more stable. Transition between these types of materials is characterized by the proposed critical composition. The critical composition for the investigated ternary non-stoichiometric system has been determined to be close to LaNi4.4Cu.