[en] Carburization and decarburization of superalloys due to the impurities in the High Temperature Gas-cooled Reactor (HTGR) helium influences the mechanical properties of the superalloys. The objective of this study is to investigate the carburization and decarburization behavior thermodynamically. The four commercial superalloys were exposed to the three simulated HTGR helium gases and the change in carbon concentration was examined. The following results were obtained: (1) The carburization and decarburization depend strongly on the superalloys and the helium gases. (2) The extent of the carburization or decarburization exhibits a linear relation against its thermodynamic driving force. The results make it possible to predict the carburization and decarburization in the HTGR helium environment. (orig.)