The hydrogen-absorbing intermetallic compound LaNi₅ was activated using volumetric apparatus (a HTP1-V volumetric hydrogen storage analyser) at 333 K. Pressure-composition isotherms (PCIs) were measured through the first 8 hydrogenation cycles and show considerable change in the hydrogen absorption behaviour through the activation process. A series of isotherms at temperatures in the range 308 – 413 K were also measured. The higher temperature measurements show a clear indication of the formation of an intermediate g phase between the solid solution a phase and b hydride phase. Van ‘t Hoff plots were determined from the measured isotherms and the average value for ΔH of 15.6 kJ/mol ½H₂ was found to be in good agreement with the literature.