We have studied the photogenerated free radicals of idebenone (IDB) and a related compound (2,3-dimethoxy-5-methyl-1,4-benzoquinone, CoQ(0)) in micellar solutions of sodium alkyl. sulfates with l-propanol (1-PrOH) addition. These radicals were also studied in 1-PrOH. UV irradiation of IDB with the long side chain in 1-PrOH produced the neutral radical, whereas in the presence of an electron donor (1,4-diazabicyclo-[2.2.2]octane, DABCO) the anion radical was formed. On the other hand, UV irradiation of CoQ(0) without the side chain produced the anion radical both in the presence and in the absence of DABCO. In micellar solutions, both compounds produced anion radicals, but the EPR (electron paramagnetic resonance) intensity of the IDB radical was very weak, even in the presence of DABCO. When 1-PrOH was added to the micellar solution, the signal intensity of the IDB radical increased approximately 7-fold. The effective electron transfer to IDB to form the radical results in a decrease of the micellar charge density. However, for CoQ(0), a significant dilution effect caused by the addition of 1-PrOH was observed. Therefore, the results obtained suggest that the benzoquinone nucleus of IDB can locate in the polar region of the micelle.