An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile (CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of P to Ni-16Cr-9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution plays a critical role in strengthening and increasing resistance to creepinduced grain boundary void formation and fracture.