The dissolution rate of hydrous oxides of iron, chromium and nickel in acid solutions was expressed as follows; v=kaH+2aAm, where aH+ is the proton activity, aA the anion acttvity, l the reaction order for proton and m the reaction order for anion.The value of l is close to 0.5 irrespective of the species of hydrous oxides but the value of m depends upon the anion species.The dissolution mechanism of hydrous oxidewas discussed from electrochemical point of view. The dissolution reaction may be regarded as consisting of the following two coupling and coupled reactions.where[Me(OH)n(Ar-)p](z+-n-rp)+ represents the intermediate metal-anion complex transferring across the Helmholtz layer at the hydrous oxide-electrolyte interface. The rate of these two reactions is controlled by the potential difference, ¢HL, across the Helmholtz layer, and the netdissolution rate of hydrous oxides can be determined by superimposing the polarization curves (potential difference, OHL, vs. reaction rate) of the two coupling and coupled reactions. The intermediate metal-anion complex has been estimated from the comparison of experimental results with theoretical derivation by assuming the Freundlich isotherm for the adsorption of anions on hydrous metal oxide. IntroductionHydrous metal oxide is a corrosion-product of metals in aqueous solutions. The hydrous metal oxide film on metals has a high corrosion resistivity in some case but accelerates the corrosion of metal in the other case. Such a behaviour of hydrous metal oxide films usually depends upon the physico-chemical properties, for example, ionic transport through the film, non-stoichiometry of the film, and semiconducting property.Hydrous metal oxide is used as an active material for the battery electrodes, for example, hydrous nickel oxide as a cathode electrode of alkaline battery. The study of acid dissolution of hydrous metal oxide is very important to understand the corrosion of metals and to examine the reactivity of active materials in batteries.Engell1) studied the acid dissolution of semiconducting metal oxide, FeO, Fe3O4 and CdO, and found that the dissolution rate depended upon the electrode-potential.He explained the potential-dependence of the dissolution rate from the electrochemical point of view, in which the following assumptions were made; 1) the transfer of ions