A method of melt-oxidation at high temperature is adopted to prepare Ir/IrO_x pH electrode, and different carbonates such as Li_2CO_3, K_2CO_3, Na_2CO_3 and their mixture were used in this process. It is shown that the (electrode) prepared in carbonate melt has good pH response characteristics, such as stability, sensitivity, response time, and anti-corrosion ability in the solution containing F~-, etc. The hysteresis mechanism was also investigated based on a storage model with a cyclic voltammetry experiment. It is found that the valence of Ir in the oxide film is +4, and the iridium oxide on the surface of the film is co-ordinate with -OH and H_2O from the result of XPS spectra. The mole fraction ratio of Ir to O is 1∶2.1 in the bulk of the oxide, but x(Ir)∶x(O) is 1∶6.78 on the surface of the oxide film by EDX technology. The hydration effect of the thermally prepared iridium oxide was investigated. Therefore, a H~+ response mechanism is suggested, being much different from the reported theory for thermally prepared iridium oxide electrode, which previously does not take the hydration of oxide into consideration. A method of melt-oxidation at high temperature is adopted to prepare Ir / IrO_x pH electrode, and different carbonates such as Li_2CO_3, K_2CO_3, Na_2CO_3 and their mixtures were used in this process. It is shown that the (electrode) prepared in carbonate melt has good pH response characteristics, such as stability, sensitivity, response time, and anti-corrosion ability in the solution containing F ~ -, etc. The hysteresis mechanism was also investigated based on a storage model with a cyclic voltammetry experiment. It is found that the valence of Ir in the oxide film is +4, and the iridium oxide on the surface of the film is co-ordinate with -OH and H_2O from the result of XPS spectra. The mole fraction ratio of Ir to O is 1: 2.1 in the bulk of the oxide, but x (Ir): x (O) is 1: 6.78 on the surface of the oxide film by EDX technology. The hydration effect of the thermally prepared iridium oxide was investigated. Thus, a H ~ + response mechanism is suggested, being much different from t he reported theory for thermally prepared iridium oxide electrode, which had does not take the hydration of oxide into consideration.