For improved efficiency and reduced emissions, the future power plants need to operate at high temperatures and pressures, which however are limited by the durability of conventional materials such as ferritic steels. Steam oxidation of a number of coatings (Al slurries, thermal spraying, chemical vapor deposition siliconizing, and nickel plating) has demonstrated the feasibility of coatings to improve oxidation resistance. Al slurry coatings combine good high-temperature oxidation resistance through the growth of an layer and the possibility to apply the coating on an industrial scale at moderate cost. This work aimed to test the oxidation performance of coatings and reference alloys in ultra-supercritical (USC) water. The tested materials included Al slurry coating on ferritic 9%Cr steel and nickel-based A263 substrates, and bulk P92, MARBN, and A263 alloys as reference specimens. Oxidation resistance was tested by exposure to flowing supercritical water (SCW) with 125 ppb dissolved oxygen at 650°C (3625 psi) up to 1000 hr.