Supercritical CO2 (sCO2) has attractive aspects as a working fluid, however, above ~550°C it carburizes conventional Fe-based alloys. Evaluations of Fe-based alloys found that advanced austenitic alloys like alloy S31025 with higher Cr and Ni contents have better sCO2 compatibility than conventional stainless steels at 650°C in sCO2 for up to 2000 h exposures. To further understand the protective scale formed on alloy S31025 in 300 bar research grade sCO2 at 650°C, the thin Cr-rich scale was analyzed after 500 h using analytical transmission electron microscopy and compared to scales formed on model Fe-20Cr-20Ni and Fe-20Cr-25Ni. Typical Mn and Si rich layers were observed but a recurring observation was an O depleted region in the scales. Characterization identified a mixed O-C phase with Cr and Fe within the scale formed on both commercial and model alloys. This phase did not form after a similar 500 h exposure at 650°C in 1 bar CO2.