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/H-2, 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 degrees 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 H-2 (g) in the same phase is theoretically feasible in acidic condition along with a relatively high pressure of CO. (C) 2015 Elsevier Ltd. All rights reserved.