The interaction of the intermetallic compound ZrNi with hydrogen was studied, and pressure-composition isotherms were obtained at 100, 200 and 250°. There was no resemblance to the zirconium-hydrogen system. The alloy formed a definite hydride at a limiting composition ZrNiHa, and possibly a second hydride at ZrNiH. X-Ray and density measurements on both the alloy and the hydrides indicated a distorted cubic structure for all phases. Hysteresis was observed on the isotherms and a possible explanation for this phenomenon is given on the assumption of lattice defects.(1) This research was supported by the Atomic Energy Commission.