Mechanical properties and fracture mechanics properties of Nickel Alloy 718 have been measured in simulated Ni/H2 cell environments to determine hydrogen environment embrittlement susceptibility. Hydrogen gas at 1000 psi with 31% KOH solution was used with controlled low oxygen concentrations to simulate the Ni/H2 cell environment. Parent material, electron beam weldments, and tungsten-inert gas weldments were tested.The most significant finding was that fatigue crack growth rates (FCGR) were strongly affected by the test environment. In particular, the concentration of trace oxygen in the hydrogen gas was important. A probable beneficial synergistic effect of KOH solution and oxygen was noted when the oxygen concentration was above 20 ppm. With trace oxygen above about 35 ppm, a significant attenuation of FCGR was observed, and FCGR's progressed at essentially the same rate as those in an air environment. With trace oxygen below about 20 ppm, FCGR's were about the same as those in pure hydrogen. Fatigue cycle frequency also had an important effect on FCGR properties.