Ultraviolet light has been found to cause carbon-oxygen bond cleavage in epoxides and carbon-sulfur bond cleavage in a thioepoxide. Studies with unsymmetrical epoxides indicated that both carbon-oxygen bonds cleave and that the intermediate diradicals are converted, by subsequent radical reactions which involve no rearrangements, into alcohols and aldehydes or ketones. For example, the irradiation of propylene oxide gave acetone, isopropyl alcohol, propionaldehyde, and propyl alcohol, though not in equal amounts. A reaction sequence is suggested which is applicable to the irradiation of epoxides and the thioepoxide. © 1964, American Chemical Society. All rights reserved.