In-situ PES measurements on various oxide thin film systems examined with MIM-type two-electrode cells revealed the variation of Fermi level upon electrochemical polarization, suggesting variations of local electrochemical redox state by the ion migration. Surprisingly narrow electrochemical windows were suggested limited by a pinning at the donor state of component cations and a chemical pinning due to oxygen evolution, in contrast to the ultra-wide band gap of these almost insulating oxides systems. The importance of the reversibility and how to control the kinetics at the anodic electrode should be emphasized. One important conclusion is the system is identical to the simple two-electrode cell composed of the electrolyte that shows variation of Fermi level within an electrochemical window of oxide system by ion migration or electrochemical reaction and modulated overpotential at electrolyte/electrode interfaces, the latter of which gives huge influence to the cell behaviors. These suggest that oxide memristor is governed by the electrochemical properties of the cell. Finally, the author proposes both a new Pourbaix-type E-pO diagram to understand the redox and acid/base reaction in addition to new proposal of the definition of basicity of complex oxide based on electronic band structure, as a necessary framework of solid state electrochemistry for oxide systems.