Investigations of the compatibility of steam generator or heat exchanger materials of a high-temperature nuclear reactor with both the primary and the secondary media of the coolant circuits were conducted. This includes studies on the metal-water reaction, the hydrogen generation involved, and the permeation of the hydrogen into the primary circuit. Permeating hydrogen can cause oxide film reduction on the primary side of the tubes and decarburization of the material. Other phenomena of interest are the possible deposition of carbon and/or the carburization of the steel by the small amounts of carbon monoxide present in the inert helium, used as coolant gas. In addition, the hydrogen permeation under low partial pressures was investigated. The hydrogen release rates (due to the metal-water reaction) were determined for several types of steels for different temperatures. The results served as a basis for an estimate of the hydrogen delivery from the secondary circuit into the primary circuit and its influence on the required gas purification capacity. An attempt is made to explain the irregularities of the hydrogen release rates observed. It appears that the carburization problem is not of major significance under the low carbon monoxide concentrations that must be expected in the coolant under normal operation conditions.