Flash photolytic investigation of photosensitizing dye “thiopyronine” was undertaken with and without redox substrates. Flash excitation of thiopyronine deaerated buffered solution without any substrate produced three transient intermediates; triplet state (Dt), half-oxidized form (Oxd) and half-reduced form or semiquinone (Red) of the dye. Red and Oxd were generated through the electron transfer reactions, Dt - Dt and Dt-D (D is a dye molecule in the ground state). Absorption spectra of these intermediates were identified and their reaction mechanisms were established. The high efficiency of this dye in the photodynamic effect was discussed in connection with the high quantum yield of triplet state production and photochemical redox reversibility of the dye.