Thermodynamic analysis of the reforming of methane with carbon dioxide alone (“dry reforming”) and with carbon dioxide and steam together (“mixed reforming”) is performed as part of a project which investigates the suitability of these endothermic reactions for the storage of solar thermal energy. The Gibbs free energy minimization method was employed to identify thermodynamically optimal operating conditions for dry reforming as well as mixed reforming with a desired H2/CO molar ratio of 2. The non-stoichiometric equilibrium model was developed using FactSage software to conduct the thermodynamic calculations for carbon formation, H2/CO ratio, CH4 conversion and H2 yield as a function of reaction temperature, pressure and reactant molar ratios. Thermodynamic calculations demonstrate that in the mixed reforming process, optimal operating conditions in a carbon-free zone are under H2O/CH4 /CO2 =1.0/1.0/0.5, p = 1 to 10 bar and T = 800 to 850℃ for the production of syngas with a H2 /CO molar ratio of 2. Under the optimal conditions, the maximum H2 yield of 88.0% is achieved at 1 bar and 850℃ with a maximum CH4 conversion of 99.3%. In the dry reforming process, a carbon formation regime is always present at a CO2/CH4 molar ratio of 1 for T = 700 1000℃ and p = 1-30 bar, whereas a carbon-free regime can be obtained at a CO2/CH4 molar ratio greater than 1.5 and T≥800℃. Thermodynamic analysis of the reforming of methane with carbon dioxide alone ( “dry reforming ”) and with carbon dioxide and steam together (“” mixed reforming ") is performed as part of a project which investigates the suitability of these endothermic reactions for the storage of solar thermal energy. The Gibbs free energy minimization method was employed to identify thermodynamically optimal operating conditions for dry reforming as well as a mixed reforming with a desired H2 / CO molar ratio of 2. The non-stoichiometric equilibrium model was developed using FactSage software to conduct the thermodynamic calculations for carbon formation, H2 / CO ratio, CH4 conversion and H2 yield as a function of reaction temperature, pressure and reactant molar ratios. Thermodynamic calculations demonstrate that in the mixed reforming process, optimal operating conditions in a carbon- free zone are under H2O / CH4 / CO2 = 1.0 / 1.0 / 0.5, p = 1 to 10 bar and T = 800 to 850 ° C for the production of syngas with a H2 / CO molar ratio of 2. Under the optimal conditions, the maximum H2 yield of 88.0% is achieved at 1 bar and 850 ° C with a maximum CH4 conversion of 99.3%. In the dry reforming process, a carbon formation regime is always present at a CO2 / CH4 molar ratio of 1 for T = 700 1000 ° C and p = 1-30 bar, while a carbon-free regime can be obtained at a CO2 / CH4 molar ratio greater than 1.5 and T ≧ 800 ° C.