A systematic study was made by gravimetry, carefully preserving the system mass zero, of the positions of the α/α+β and α+β/β phase boundaries and the plateau lengths in LaNi5–H, while absorption–desorption cycling under various conditions of temperature and pressure. Our findings include: (i) in agreement with previous studies, the initial capacity of the virgin intermetallic is in excess of H/M=1.1; (ii) however, the initial desorption plateau is shorter than the initial absorption plateau by nearly 0.1 in H/M, owing to the trapping of H, possibly in lattice defects; (iii) absorption–desorption cycling at any temperature in impure hydrogen shortens the plateaux (capacity degradation), greatly exacerbated by elevated temperature; (iv) however, the total H capacity remains relatively constant, with an exchange of H between ‘reversible’ sites and trap sites, manifest as the drift in the α/α+β phase boundary noted above. The reality of the H trapping owing to activation has been proved by independent techniques, such as deep-inelastic neutron scattering. It is unlikely that this H is initially trapped in the form of LaHx, although this and other La–Ni hydrides do form during extended cycling at elevated temperatures.