Two types of defects are formed when crystalline NaClO3 and KClO3 are photolyzed at 26 K with monochromatic light of a wavelength corresponding to the fundamental absorption. One type of defects absorbs around 380 nm and has an ESR spectrum characteristic of trapped O2. The other type of defects absorbs around 270 and 580 nm and has no ESR spectrum. The two types of defects are interconverted reversibly by bleaching within the respective absorption bands. Chemical analysis and isotopic labeling indicates that the defects consist of fragments of a single chlorate ion. The 380 nm absorption is assigned to charge transfer in the complex [ClO−,O2]; the 270 and 580 nm absorptions are assigned to [Cl−,O3]. Both the optical spectra and the mechanism of photolysis are represented by a model based on the enthalpies of formation of various configurations of one chlorine and three oxygen atoms.