Slow strain-rate tensile testing of Ni-based Alloys X-750 and 625 was performed in high-purity, deaerated water in order to determine whether hydrogen embrittlement occurs in these alloys at room temperature and 288°C. These tests were conducted at an initial strain-rate of 4.6×10-7/sec under both 30 psig nitrogen (0 cc H2/kg H2O STP) and 40 psig hydrogen (60 cc H2/kg H2O STP), on Alloy X-750 in two heat-treatment conditions and on Alloy 625 in a solution annealed and aged condition. At room temperature in the hydrogenated environment, both alloys showed a pronounced susceptibility to hydrogen embrittlement. The presence of hydrogen reduced both the elongation to failure and the reduction in area at fracture by about 50%. Fractography indicated a transition from ductile, transgranular failure in the nitrogen environment to predominantly intergranular fracture under hydrogenated conditions. Fractography of the specimens tested at room temperature showed a transition in crack growth behavior from mixed mode slip band decohesion in nitrogen environments to Mode I intergranular fracture path in hydrogenated environments. In contrast, hydrogen had no discernible effect on the measured properties at 288°C under the slow strain-rate conditions imposed.