Alloxanthine-inhibited xanthine oxidase (XOD) was found to be photoreactivated by irradiation of light of wavelengths in the range of 340–430 nm. The enzyme activity can be fully controlled to be on or off by many dark–light cycles. Electron spin resonance measurement shows the appearance of the molybdenum (V) ion and the reduced form of flavin adenine dinucleotide (FADH^·) radical signals after irradiation of the alloxanthine–XOD complex. Electronic-absorption spectrum also shows the bleaching of Fe/S and flavin adenine dinucleotide chromophores at 375 and 450 nm as well as broad-band absorption of FADH^· in the range of 500–700 nm. The quantum yield of photoreactivation of the enzyme activity is ∼0.06. A photoinduced intraenzyme electron-transfer model is proposed to rationalize the photoreactivation process.