Steels have a proven track record of safe operation in steam power plants for decades. Interest in developing supercritical CO 2 power cycles as a more efficient and sustainable alternative to steam cycles has driven a need to understand steel performance in these new environments. In particular, the potential of the high temperature CO 2 environment to influence the creep behavior of the steel must be determined. Prior research on this topic between the 1960s and 1980s found conflicting conclusions, but nevertheless raised the possibility that carburization during CO 2 exposure may strongly affect the creep behavior. This raises concerns particularly for thin-sectioned components such as compact heat exchangers, where even small rates of carburization can become problematic over long operating lifetimes. To shed light on this issue, this research investigates the creep behavior of austenitic stainless steel 347H and 309H (a higher Cr alternative) at 650°C. Specimens of 0.5, 1.0, and 2.0 mm thickness were tested to further assess the effect of steel thickness. Both steels show a reduction in creep life in CO 2 relative to air, with 309H showing slightly better performance than 374H. Analysis is ongoing to determine the reason for degraded creep properties.