A theory of the electrode potential of nonstoichiometric electron conductive metal oxides MeO_n on a foreign metal is presented. When the oxygen ions of the oxide are in equilibrium with the electrolyte the oxide electrode potential is epsilon<i>h</i> = (1/2F) . dDELTAG (<i>n</i>)/<i>dn</i>-0.059 . pH with the free enthalpy DELTAG (<i>n</i>) for the reaction Me + <i>n</i> H₂O right arrow MeO_n + <i>n* 1H₂ as a function of the degree of oxidation <i>n</i>. The oxide electrode potential for the case of an equilibrium of the metal ions between the oxide and the electolyte is also derived. The condition of equilibrium between the oxide MeO_n and the electrolyte with dissolved z-valent metal ions (<i>z</i> + 2<i>n</i>) is given. The equilibrium oxide relative to its parent metal and the corresponding potential are discussed. The potential of a passive electrode depends only on the surface oxide in contact with the electrolyte and is independent of the construction of the passive layer. The effect of the corrosion rate and of the current density on the passive potential and on the construction and composition of the inner layer are discussed. The establishment of a stationary layer is explained. The experimental pH dependence of the corrosion current density of equal surface oxides of Ni, Fe, and Cr are analyzed. For this reason passive potentials against the hydrogen electrode at the same pH value are compared. -- CCI