The lumiflavin‐sensitized photooxygenation of indole in aqueous solutions has been investigated by means of steady light photolysis and flash photolysis. The semiquinone of lumiflavin and the half‐oxidized radical of indole were formed by the reaction between triplet lumiflavin and indole (3.7 times 10⁹M^‐1 s^‐1). The semiquinone anion radical of lumiflavin reacted with oxygen to form superoxide radical. The triplet state of lumiflavin also reacted with oxygen forming singlet oxygen, ¹O₂. But the reaction between ¹O₂ and indole (7 times 10⁷ M^_l s^_1; estimated from steady light photolysis using Rose Bengal as a sensitizer) was far less efficient than the reaction between indole and triplet lumiflavin. The quantum yield of the lumiflavin‐sensitized photooxygenation of dilute indole via radical processes was much higher than that via¹O₂ processes, though appreciable ¹O₂ was formed.