This research supports efforts to design and build safe and cost-effective advanced ultra-supercritical (A-USC) steam boilers and turbines to increase energy efficiency and reduce emissions, including CO2. A flowing steam autoclave, rated to 346 bar/704°C or 228 bar/760°C, was constructed. Comparison tests were run at 1 bar. Parabolic rate constants were estimated from mass change data and compared with literature values—but none at this combination of high of temperature and pressure. Test materials included austenitic stainless steels 304H, 347H and E-Brite, and Ni-base alloys H230, H263, H282, IN617, IN625 and IN740. The 1 bar data matched literature values for chromia in steam, while the high pressure data had a higher oxidation rates. At high pressures the oxide scales were thicker, and for most alloys the parabolic rate constants were one–to-two orders of magnitude higher than at 1 bar. Possible causes for increased oxidation rates with increased pressure were examined.