In situ x‐ray diffraction (XRD) technique was employed to study unstable phases formed in sintered or anodically oxidized tungsten and molybdenum trioxide film electrodes under electrocoloration conditions in , in order to discuss their electrochromic characteristics on the basis of structural properties. The XRD patterns for tungsten trioxide film electrodes revealed that was transformed to hydrogen bronzes of in company with coloration while no structural change was observed in the anodic oxide film of . These findings supported that the electrocoloration proceeds according to the following processes: and . For the sintered film electrode, the crystal lattice was discontinuously expanded in the direction perpendicular to layers formed by octahedra by insertion of proton to the layer under electrocoloration condition: . The anodic oxide film formed on a Mo plate had been already colored black‐blue, but it provided a different XRD pattern by cathodic polarization at −0.30V.