Power cycles using supercritical CO₂ (sCO₂) as the working fluid are expected to offer significantly improved conversion efficiency and lower levelized cost of electricity over current power systems. In addition, sCO₂ power cycles operated in a direct-fired configuration (where the combustion gases are used to turn the turbine) may allow for capture of 100% of the CO₂ produced by combustion at almost no additional energy penalty. A broad materials evaluation effort is underway at the National Energy Technology Laboratory (NETL) to help enable direct-fired sCO₂ power cycles as a promising next-generation energy conversion technology. Of primary interest is the corrosion behavior of structural alloys in the complex multi-oxidant environments expected for this application. In this study, the corrosion behavior of several candidate structural alloys was evaluated by simulating the conditions expected for the high-temperature regime of a direct sCO₂ power cycle. Several commercial Fe (grades 22 and 91, 304, 310, 347H) and Ni (617, 230, 625, 740H, 282, 263) alloys were exposed at 1 atm and 550 °C (Fe alloys) or 750 °C (Ni alloys) to CO₂ containing 4 vol% H₂O and 1 vol% O₂, with and without 0.1 vol% SO₂. The alloys were massed at 500 h increments up to 2500 h to determine the oxidation kinetics. At 2500 h samples were removed from the test and characterized by XRD, SEM, and EDS, while duplicate samples continue to be exposed for longer times to assess for possible breakaway oxidation. Ni alloys all exhibited relatively low oxidation rates to 2500 h (<1 mg/cm² in all cases) and XRD and SEM analyses revealed that Cr-rich oxides were the primary reaction products. For Fe alloys (current exposure time of 1500 h with tests ongoing), 304, 310, and 347H show minimal mass gains (<0.2 mg/cm²), while grade 91 and 22 show similar and more substantial mass gains (≈10 mg/cm²). In general, the presence of SO₂ led to approximately twice the mass gain as the SO₂-free condition for both Ni and Fe alloys. The results are used to assess the influence of impurities on the oxidation behavior of structural alloys in high-temperature CO₂, and to evaluate the suitability of these alloys for direct sCO₂ power cycles.