Structural changes of La2Ni7H, during the first and second absorption desorption processes along the P C isotherm were investigated by in situ X-ray diffraction (XRD). Orthorhombic (Pbcn) and monoclinic (C2/c) hydrides coexisted in the first absorption plateau,. but only a monoclinic (C2/c) hydride was observed in the first desorption plateau. Phase transformation of La2Ni7Hx was irreversible. between the first as well as the second absorption desorption, process.. The lattice parameters and expansion of La2Ni4 and LaNis cells during the-absorption-7desorptiowproCess were refined using the Rictveld method. The lattice parameter's a and b ofthe.orthorhornbic hydride (Pbcn) decreased, while the lattice, parameter c. increased : with. increasing' hydrogen content in the first absorption. During the first absorption, the volume of the orthorhombic La2Ni4 cell expanded by more than 50%, while the expansion of the LaNis cell was below 10%: The monoclinic La2Ni4 cell expanded to approximately four times the size of the LaNis cell in the first absorption. The lattice parameters a, b, and c of the ;monoclinic hydride (C2/c). decreased with decreasing hydrogen content in the first. desorption contracted isotropically in the first desorption. These La2Ni4 and LaNis cells