The intramolecular electron transfer reaction induced by light irradiation of flavin-adenine dinucleotide (FAD) was studied by an action spectra of the magnetic field effect on the transient absorption. The action spectra depended on pH in the region of 2.0-4.0. They are reproduced by the sum of two components: T-T absorption and radical form of flavin. The contribution of T-T absorption is dominant at pH less than or equal to 2.3 whereas that of flavin radical is dominant at pH > 3.3. The existence of the equilibrium between the radical pair and triplet excited state is concluded. (C) 2002 Elsevier Science B.V. All rights reserved.