Abstract This work is a predictive thermodynamic study focused on competing reduction processes in molten carbonates, involving the following redox systems CO2/CO, CO2/C, CO/C, H2O/H2, M+/M (M = Li, Na or K). Its main purpose is to determine the best electrolyte and operating conditions relative to the CO2 reduction into CO or C between 450 and 750 °C, from a theoretical point of view. Potential-oxoacidity diagrams are established at different temperatures for binary eutectics (Li2CO3-K2CO3: 42.7–57.3 mol.%, Li2CO3-K2CO3: 62–38 mol.%, Li2CO3-Na2CO3: 52–48 mol.% and Na2CO3-K2CO3: 56–44 mol.%), and for the ternary carbonate eutectic Li2CO3-Na2CO3-K2CO3: 43.5–31.5–25 mol.%. As residual water can be present, its reduction into hydrogen is studied as well. The coexistence of CO (g) and H2 (g) in the same phase is theoretically feasible in acidic condition along with a relatively high pressure of CO.