As generally recognized silica is an inert material and therefore chemically stable. Recently, however, there has been increased number of the reports that the chemical activity of silica generated when silica is treated by evacuation at a high temperature. With this regard, the present report is an overview of our work on photocatalysis by silica. The evacu-ation at a high temperature brings about a generation of an active site; possibly an abnormally coordinated silicon atom that is a photoluminescence center as well as a paramagnetic radical center. When the site is in contact with oxygen gas, an O₂^-anion radical forms by UV light irradiation, which can oxidize alkene molecules and carbon monoxide. In the absence of oxygen, by introduction of alkene molecules to the system involving UV-irradiated silica, alkene metathesis takes place. The metathesis reaction proceeds via. a metalacyclobutane intermediate although the presence of carbene species has not been confirmed yet.