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Abstract—The triplet excited states of four derivatives of ubiquinone-30, in which various ring substituents are progressively altered, have been studied by laser flash photolysis (265 nm) and pulse radiolysis (9–12 MeV electrons). Triplet absorption spectrum, extinction coefficient, lifetime, energy level and quantum efficiency of formation were determined. By comparison with previous studies with ubiquinone-30, it is deduced that the low triplet energy and quantum efficiency of formation of triplet ubiquinone-30 is caused by the presence of the two adjacent methoxy substituents, rather than to the isoprenoid side chain. The low quantum efficiency of triplet formation, although consistent with in vivo ubiquinone photomodification occurring via the triplet, suggests that little of the ubisemiquinone observed in bacterial photosynthesis is formed via excited ubiquinone.