This paper proposed CO₂ mixtures as working fluids in closed Brayton power cycles using flue gases at relatively high temperature (400-450 °C) as waste heat source. Firstly, a comprehensive selection criterion is defined for choice of working fluids to be employed as additives in CO2 mixtures. Secondly, the thermodynamic properties of the mixtures are calculated at different molar compositions using an appropriate equation of state. The binary interaction parameters involved in the equation of state are obtained with the help of available experimental VLE data or by estimation method in case of non-availability of the VLE data. As a benchmark, the study also investigates the thermodynamic performance of advanced sCO₂ cycle layouts to compare with the performance of cycles operating with CO₂ mixtures. Sensitivity analysis reveals that the power cycles operating with CO₂-Novec5110 mixture (with 0.2 mole fraction of Novec fluids) show 3 percentage points rise in cycle thermodynamic efficiency (0.219 versus 0.252) with lower cycle operating pressures as compared to recuperative with mass split sCO₂ cycle. In case of CO₂-R134a mixture working fluid (with 0.3 mole fraction of R134a), total efficiency of about 0.15 is obtained at cycle maximum pressure of 200 bars compared to simple recuperative sCO₂ cycle with total efficiency of 0.13 at rather higher maximum pressure of 400 bars.