Electrochemicalreactions at semiconductor surfaces depend on transport of electrons or holes between the surface and the bulk. This is very similar to transport in metal‐semiconductor Schottky barriers and leads to differences between n‐ and p‐type materials and to photochemical effects. Reaction at the surface may involve either decomposition of the semiconductor itself, or exchange of charge with an ion in the electrolyte. Photochemical decomposition of the crystal is common among binary compound semiconductors having pronounced ionic character. In an elementary model for this reaction the negative ion is identified with the valence band and the positive ion with the conduction band. Ions of one sign are neutralized, forming either a gas or a solid deposit on the surface. Minority carriers are needed to complete the reaction and these are generated by the light. Covalent semiconductors can also undergo reactions that are limited by the supply of minority carriers. Reactions with ions in solution can be understood by regarding the ions as localized states for electrons. Different ionic species have different energy levels for electrons and can react with carriers from either the valence band or the conduction band.