Catalysis by oxides concern mild selective oxidation as well as total oxidation reactions. Oxides are all semiconductors, either pure, mixed, doped or supported and the redox reactions they catalyse are all connected with their electronic properties. n-Type semiconductors possess anionic vacancies which, when once or twice ionized, are the real oxidative agent of the oxide catalyst. This is illustrated by the Sn-Sb-O system. p-Type oxide semiconductors have positive holes h(+) as charge carriers, associated with an excess of anionic oxygen. They are illustrated by the case study of vanadyl-pyrophosphate (VO)(2)P2O7 able to selectively oxidize butane into maleic anhydride in a single pass. In situ measurements of the electrical conductivity of titania during the catalytic oxidation of CO clearly indicated that the oxidizing agent was O- species. All what is described in thermal catalysis can be transposed to photocatalysis at room temperature, with the simultaneous formation of photoelectrons and holes. In dry medium (gas or liquid phase), selective mild oxidation occur involving a neutral activated species O-*, illustrated by the direct oxidation of 4-tert-butyl-toluene into 4- tert-butyl -benzaldehyde. (c) 2005 Elsevier B.V. All rights reserved.