To evaluate performance of materials in a high-temperature reactor, Dragon Project has conducted creep/corrosion tests in air and in impure helium on a wide range of structural and experimental steels and high-temperature alloys. These included 11 casts of austenitic steels tested in helium with impurity levels controlled at 50 to 100 jut at H2, 25 to 50 µ at CO, 3 to 8 µ at CH4, and0.5 to 3 µ at H2O in a total pressure of 1.8 atm. Tests were conducted at 650 to 800°C for times up to 15 000 h. For materials based on 9 to 17% nickel and 15 to 18% chromium, surface corrosion rates were lower in steels containing 0.16 to 0.7% niobium than in those with similar levels of titanium or those of AISI Type 316 stainless steel. Subsurface intergranular oxidation and carburization were also noted in the niobium-free steels. Depths of intergranular oxidation ranged up to 200 µm, depending on strain, time, and temperature. In AISI Type 316 stainless steel, carburization was noted for depths up to 1.3 mm after 10 000 h at 750°C. Results on four casts of Alloy 800 have been described. Usually these displayed low oxide growth rates and some intergranular oxidation, but in one cast subjected to a nonstandard heat treatment (vacuum annealing in silica capsules), there was a tendency for thick patchy oxidation associated with deep oxide penetrations and local carburization. Creep and rupture strengths in helium were usually at least as good as those in air, and there was no evidence that the intergranular oxidation or subsurface carburization, seen in some steels at 700°C and above, caused premature creep cracking. The one exception to this behavior was again the batch of Alloy 800 subjected to the detrimental heat treatment. At 650 there were some cases of low ductility failures in Alloy 800, but these were probably caused by inherent material behavior rather than any adverse effect of helium. However, some preliminary data suggest that crack nucleation in Alloy 800 is easier in air but that propagation is faster in helium.