Apart from the electrocatalytic activity of the metal-electrolyte contact, the conversion ef¢ciency of photoelectrochemical cells depends on the extent of the recombination rate of photogenerated charge carriers at interface states with respect to the rate of charge transfer from the semiconductor across the interfacial layers to the electrolyte. The formation of metal-semiconductor junctions by electrochemical deposition is accompanied by often-unavoidable lateral surface chemical and electrochemical reactions that lead to the formation of localized energy states closely related to the chemical transformation of the surface structure of the semiconductor. Thus, the selection of plating conditions for controlling the level of arising pernicious interface states requires a better knowledge of chemical and electronic behavior of the semiconductor-electrolyte interface. In this chapter, combined electrochemical and spectroscopic analysis of surface states will be presented with emphasis on the analysis of the Si-SiOx interface as a model system.