Compressibility of hydrogen in Ni and Re hosts at pressures to 123 GPa

W-H and Re-H binary systems have been studied at room temperature by X-ray diffraction in a diamond - anvil cell. The formation of the hydrides NiH an d ReH0.4 with no change in the types of the host metal lattices was observe d through the expansion of the host lattices. No structure changes were obs erved in the metal sub-lattice of either hydride under increasing pressure and the equations of state have been obtained to 123 GPa. The difference in the partial hydrogen volume as a function of pressure between the Ni-H and Re-H systems can be understood with reference to the behaviour of conducti on electrons. The pressure dependence of the partial hydrogen volume in the se hydrides supports the hypothesis of the apparent common-volume behaviour of hydrogen in a metallic environment.