Abstract These initial results strongly suggest that the enrichment and segregation of niobium at grain boundary surfaces played an important role in the environmental enhancement of creep crack growth in the Inconel 718. The precise mechanism, however, still needs to be established. Available evidence suggests that the mechanism involves the formation and fracture of a brittle niobium oxide film on the boundary surfaces which reduces the alloy's crack growth resistance. Oxidation and decomposition of NbC particles at the crack tip surfaces and at the grain boundaries appear to be the principal source of niobium. The segregated niobium (in the form of Ni3Nb precipitates) at the grain boundaries is expected to also contribute to the embrittlement. The source of oxygen would be that of an external (pseudo-equilibrium) surface oxide. The mechanism would be consistent with the observed independence on oxygen pressure from 2.67 to 100 kPa. The rate controlling process would be identified with the reactions of oxygen with niobium carbides (266 versus 287 kJ/mol) or with the segregated niobium. The lower activation energy for crack growth in moist argon and air (about 190 kJ/mol) still needs to be understood. It may reflect the additional embrittlement contribution from hydrogen, and a transfer of the rate controlling process. Further studies are underway to critically examine this and other hypotheses in developing a more complete understanding. The important role of niobium in the enhancement of creep crack growth in nickel-base superalloys is demonstrated by the correlation between environmental sensitivity and niobium concentration in a series of commercial alloys. The detailed aspects of the interactions with niobium are under investigation and will be reported later.