Abstract The relationship between the hydride phases in the LaNi 5-H 2 system was investigated using in situ X-ray diffraction measurements in hydrogen atmospheres at pressures of up to 5 MPa. A new hydride phase, denoted the γ hydride here, was observed in addition to the α and β phases in the region of the reaction plateau in the temperature range 40–95 °C. These three phases usually coexisted under these conditions. A single phase of the γ hydride was not observed but its X-ray diffraction spectrum was synthesized by computer to determine the crystal structure. The γ phase was indexed according to a hexagonal lattice with a = 5.28 A ̊ , c = 4.04 A ̊ and its composition was estimated to be close to LaNi 5H 4. The γ hydride was obviously different from LaNi 5H 6 which was generally believed to be the only hydride phase of the LaNi 5-H 2 system in the pressure range usually used. The hydrogen absorption-desorption cycles had a marked effect on the stability of the γ hydride. After many cycles the γ phase became more stable.