Carburization of high-temperature alloys has been frequently observed during exposure to dry high-temperature gas-cooled reactor (HTGR) helium compositions. Therefore, the influence of carburization on mechanical properties of alloys that may be used for HTGR high-temperature components has been studied. In creep rupture tests on high-temperature alloys for up to 20 000 h, the data in air and in various simulated HTGR heliums lie in the same scatterband irrespective of carburization that has been observed in the contaminated helium atmospheres. The dependence of room temperature tensile properties and the impact strength in the 20 to 800 °C range on the carburization level has been measured so that the maximum carbon level for a given room temperature ductility and impact strength could be specified. The results showed that the minimum room temperature elongation fell to below 5% when the carbon content exceeded 0.5 wt% for Incoloy-800H and 0.2 wt% for lnconel-617. At these carbon levels, the alloys have impact strengths (ISO V-notch specimens) of ∼50 J or above at temperatures in the 25 to 800°C range.