[en] During the reporting period controlled impurity helium and air creep-corrosion tests on the original (Group I) 17 alloys in the program reached a maximum 14,000 hr duration, and similar tests on 9 additional (Group II) alloys reached a maximum of 5000 hr. The parallel unstressed controlled impurity helium and air fracture behavior tests on the same alloys reached a maximum of 10,000 hr. The initial tests in the parametric helium impurity corrosion study, which are investigating the effect of methane variation, were initiated and exceeded 1000 hr duration. The results of detailed metallurgical analyses and post-exposure tensile testing on the Group II alloys after 3000 hr stressed and unstressed exposure in controlled impurity helium at 800 to 1000⁰C are reported. Data for the wrought austenitic alloys Hastelloy X (new heat), Hastelloy S, RA 333, and HD 556 and the cast austenitic alloys HK 40, Supertherm, Manaurite 36X, Manaurite 36XS, and Manaurite 900 are reported. All alloys carburized significantly at 900 and 1000⁰C, with minor increases of carbon content exhibited at 800⁰C. The alloys were grouped with respect to the morphology of their oxide/carbide corrosion scales and the composition of these scales as identified by SEM microprobe analyses. Generally, carburization depths at 1000⁰C were greater in the wrought alloys than in the cast alloys. Room-temperature tensile properties of the wrought alloys were more severely affected by impure helium exposure than were the tensile properties of the helium-exposed cast alloys. Decreases in ductility of the exposed wrought alloys, presumably caused by carburization, were greater than would be expected from thermal aging alone