Materials for high-temperature gas-cooled reactor (HTGR) application are selected according to their creep behavior. For two alloys—Incoloy-800 used for the live steam tubing of the thorium high-temperature reactor and Inconel-617 evaluated for tubings in advanced HTGRs—creep curves are measured and described by equations. A microstructural interpretation is given. An essential result is that nonstable microstructures determine the creep behavior.