Stress corrosion crack growth rate (SCCGR) tests of nickel alloys were conducted at 338°C and 360°C as a function of the hydrogen concentration in high purity water. Test results identified up to a 7x effect of hydrogen levels in the water on crack growth rate, where the lowest growth rates were associated with the highest hydrogen levels. At 338°C, the crack growth rate decreased as the hydrogen levels were increased. However, different results were observed for the test conducted at 360°C. As the hydrogen level was increased in the 360°C tests, the crack growth rate initially increased, a maximum was exhibited at a hydrogen level of -20 scc/kg, and thereafter the crack growth rate decreased. Based on this testing and a review of the commercial literature, the thermodynamic stability of nickel oxide, not the dissolved hydrogen concentration, was identified as a fundamental parameter influencing the susceptibility of nickel alloys to SCC. These test results are discussed in relation to the accuracy of extrapolating high temperature SCC results to lower temperatures. Keywords: SCC, nickel alloy, dissolved hydrogen, electrochemical corrosion potential