The concept of surface impedanceZ E of an electric ac-fieldE ⊥ perpendicular to metal surfaces is developed and then applied to clean and oxide coated metals. Whereas clean metals show values forZ E which are not detectable, yet; oxide coated metals show very much enhancedZ E . This enhancedZ E corresponds to shielding ofE ⊥ inside the oxide and to rflossesR E ≳10−6Ω, which could explain the losses of superconducting cavities. The reason for enhancedZ E are localized electron states in the oxide neighboring the metal, which give rise to a coupling of conduction electron to localized electron states.