Ni-base alloys are of increased technological interest for power generation in advanced steam supercritical carbon dioxideas well as their traditional role in gas turbines. For applications at650°-800°Ccommercial Ni-base alloys range from solid solution strengthened alloys to precipitation strengthened alloys with 0-2% additionsof Al and Ti. In order to understand the effect of environment and composition on their oxidation behavior commercial and model Ni-(18-22%)Cr alloys were exposed to laboratory air air with 10%H2O and steam at 800°C and supercritical CO2 at 700°C. Of particular interest is the effect of environment on scale thickness and the depth of internal oxidation of Al and Ti. For 17bar steam at 800°C the depth of internal attack increased with increasing Al+Ti content and a time series of specimens was conducted for up to 10000h to quantify the rates. Research sponsored by the U.S.Department of Energy Office of Fossil Energy Crosscutting Research Program.