Abstract: EDX micro-analysis method was used to characterize the ratio of Ir to O in the oxide film prepared by thermal-oxidization.XPS was conducted to analyse the elements in the oxide and the valence transformation of Ir element in the H+ involved reaction as well as the existing forms of O element.The mole fraction ratio of Ir to O exceeded 1∶6 at the aged oxide film surface,while it nearly retained 1∶2 in the bulk of the film,so it was believed that the electrode surface was composed of hydrous iridium oxide based on the EDX determination.The XPS results showed that Ir4+ transformed to Ir3+ in the electrode reaction,and O element existed in the forms of O2-,-OH,H2O,and thus a thermodynamic mechanism of H+ response was suggested to the hydrous iridium oxide electrode.EIS experiment data suggested that the dynamic process of the H+ response was controlled by the electrochemical reaction on the electrode.