Creep rupture tests on Incoloy-800H and Hastelloy-X have been carried out in a simulated steam-cycle high-temperature gas-cooled reactor helium environment containing a high level of moisture. For the maximum test time of 16 000 h, the creep rupture behavior of lncoloy-800H in the helium environment was not significantly different from that in air. Hastelloy-X showed a slightly lower rupture life in helium in comparison with standard air tests. Surface cracks initiated at the grain boundaries penetrating surface and cavities formed at the grain boundary triple points have been identified as major fracture mechanisms in both environments. Oxidation was the only gas/metal interaction observed in the helium environment.