In the High-temperature Gas-cooled Reactor (HTGR), structural metals will be required to operate at temperatures of 1000 to 1400 deg F in an impure- helium environment for extended periods of time. Six candidate materials for such applications, including nickel alloys and stainless steels, were subjected to simulated reactor environments for times up to 3000 hr to evaluate the extent of corrosion and the changes in mechanical properties occasioned by I such exposures. Under the experimental exposure conditions (helium containing 200 to 300 ppm CO and 200 to 300 ppm H/sub 2/, or helium containing 2000 to 3000 ppm CO and 2000 to 3000 ppm H/sub 2/, at temperatures of 1200 or 1400 deg F), oxidation rather than carburization appears to be the major corrosion process; however, in some cases alternating carburization and decarburization were observed. Measurements of weight gains are described. None of the materials studied exhibited a large change in their mechanical properties after exposures at 1200 deg F. After exposures at 1400 deg F, moderate changes in mechanical properties were observed for all materials except Type 321H stainless steel, which exhibited a large decrease in ductility after l500-hr exposure to the higher impurity concentration. In general, the control specimens were affected as greatly as the specimens exposed to the impure helium environments, under the conditions of these tests. (R.J.S.).