The chemical-state changes in the reduction process of metal chlorides (CoCl2, NiCl2, and CuCl2) supported on SiO2 were clarified using in situ XAFS measurements. Hydrates were supported in the dried samples prepared by the incipient wetness impregnation, and all metal chlorides changed to anhydrides by heating to 200 degrees C under an H2 gas flow diluted by He. Subsequently, CoCl2 and NiCl2 were reduced to metallic Co and metallic Ni, respectively, while CuCl2 was reduced to metallic Cu via CuCl. The temperature at which chloride ions dissociate from anhydrous metal chloride particles was directly determined. In addition, the conversion temperature was determined by in situ XAFS analysis of the process in which metal particles on SiO2 are oxidized (chlorinated) by the Cl2 gas diluted by N2 at elevated temperature. Finally, the thermochemical redox temperature between metal oxides and metals was found to be correlated to their electrochemical conversion voltages. Based on this correlation, the electrochemical conversion voltage was predicted from the thermochemical conversion temperature of the MCl2/M couple determined in this study.