In order to estimate the influence of the helium pressure on the corrosion of ferritic and austenitic materials, tests were carried out under 2 absolute bar in a circuit without helium recirculation and under 50 bar in the AIDA loop. In both cases the partial pressures of impurities were 1.500, 50, 450, and 50 μatm for H2, H2O, CO, and CH4, respectively. The interruption of the French high-temperature gas-cooled reactor R&D program has only produced limited results: 1. At 650°C the behavior of 11% chromium ferritic steel HT 9, Types 304 and 316 austenitic steels, and Incoloy Alloy 800H is excellent; the oxidation rates are low and decrease with time. 2. At 750 and 870°C, Hastelloy-X offers better resistance to external and intergranular oxidation than alloys 800H and Inconel-617. 3. At these three temperatures, the oxidation kinetics are appreciably faster under a pressure of 50 bar than under 2 bar. 4. Whereas carbon steel is subject to decarburization at 550°C, a carburization phenomenon is observed for alloys 800H, Inconel-617, and Hastelloy-X at 750 and especially at 870°C. 5. As for the influence of the initial surface preparation, mechanically polished specimens generally present a lower oxidation rate than those polished electrochemically.