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The sulfur chemistry in oxyfuel combustion systems has received growing attention lately. The formation of SO 3 is of special concern, because of the elevated SO 2 concentrations found in oxyfuel, compared to air-fuel conditions. The present study focuses on the gas-phase chemistry and examines the impact of different combustion parameters and atmospheres on the formation of SO 3 in oxyfuel and air-fuel flames, using a detailed gas-phase model. The work also includes a summary of the presently available SO x data from experiments in laboratory and pilot-scale combustors. The reviewed experimental data, as well as the modeling results, show significantly increased SO 3 concentrations in oxyfuel, compared to air-fuel conditions. The modeling results reveal a complex behavior of the SO 3 formation, which is influenced by direct and indirect effects of the SO 2 , O 2 , NO x , and CO content in the flue gas. One of the main contributors to the increased SO 3 concentration in oxyfuel, compared to air-fuel conditions, is the high concentration of SO 2 in oxyfuel combustion. The modeling also shows that the stoichiometry, residence time, and flue-gas cooling rate are critical to the SO 3 formation. Thus, in addition to the stoichiometry of the flame, the flue-gas recycling conditions are likely to influence the formation of SO 3 in oxyfuel combustion.