This project aims to investigate the corrosion behavior of carbon steel in supercritical and subcritical CO2 in the presence of O2 as a contaminant. The contaminant concentration corresponds to the amount of O2 that can be present in CO2 from in particular an oxyfuel combustion flue gas. The effect of oxygen (4 vol.%) on the corrosion performance of mild steel (UNS G10180) in CO2-saturated brine was investigated using a 4-liter autoclave in experiments of 48 hours duration. Experiments were conducted at 25°C and 80°C at CO2 partial pressures of 4 MPa and 9 MPa this corresponds to subcritical-supercritical boundary conditions that encompass three different phases of CO2 – gaseous liquid and supercritical. Electrochemical measurements weight loss determination and corrosion product characterization using a multi-pronged analytical approach were conducted. Severe corrosion was observed in the steel specimen after 48 hours of exposure at 25°C with the presence of O2 that accelerated the corrosion reaction. Experiments at 80°C revealed bilayers of corrosion products on the steel surface which consisted of iron carbonate iron oxides and iron oxyhydroxides. Localized corrosion was only observed at 80°C and 4 MPa CO2 in the presence of O2.