In this study, the performance of a SBC (supercritical gas Brayton cycle) using CO2-based binary mixtures as the working fluids have been studied. Based on the thermodynamic analyses, an in-house code has been developed to determine the cycle efficiency and the amounts of heat transfer in the HTR (high temperature recuperator) and the LTR (low temperature recuperator) with different CO2/additive gas ratios. Several gases are selected as potential additives, including O-2, He, Ar, Kr, butane and cyclohexane. Compared with the Brayton cycle with pure S-O-2 (supercritical carbon dioxide) as the working fluid, it is found that both CO2 He and CO2 Kr mixtures can improve the thermodynamic performances of the SBC by increasing the cycle efficiency and decreasing the amounts of heat transfer in the HTR and LTR. For the cycles with the pure S-O-2 mixture, CO2 butane mixture and CO2 cyclohexane mixture as the working fluids, the cycle efficiencies decrease with increasing main compressor inlet temperature. However, when the main compressor inlet temperature is above the critical temperature of pure CO2, the cycle efficiencies of the cycles with CO2 butane mixture and CO2 cyclohexane mixture are higher than that of the cycle with pure CO2 as the working fluid. For the cycles with CO2-based binary mixtures and pure S-O-2 as the working fluids, the higher reactor outlet temperature always results into higher cycle efficiencies and larger amount of heat transfer in the HTR and smaller amount of heat transfer in the LTR. (C) 2015 Elsevier Ltd. All rights reserved.