Abstract The photosynthetic electron transport inhibitor dibromothymoquinone was found to mediate the photoreduction of horse heart cytochrome c in aqueous solution. This process appears to involve the reduction of dibromothymoquinone to the corresponding hydroquinone followed by direct electron transfer to cytochrome c. A subsequent‘autoxidation’of the reduced cytochrome c takes place which requires the presence of both oxygen and quinone. Irradiation of ethanolic stock solutions of dibromothymoquinone generates a hydroquinone species which is (1) capable of reducing cytochrome c directly in aqueous solution in the dark and (2) chemically distinct from the parent compound. This photore-duced species is stabilized in ethanolic solution but undergoes autoxidation (t1/2= 24 s) when placed in an aqueous medium. Because dibromothymoquinone is used in photosynthetic studies, some care must be taken to avoid complications which might arise from the above reactions; suitable precautions are discussed.