The decay process of the radical pair produced in the photoreduction of anthraquinone in micellar solutions of sodium dodecyl sulfate (SDS) at various concentrations was observed by the pulse-mode product-yield-detected ESR technique. The rate of separation of the two components of the radical pair (k(ESC)), SDS radical and anthrasemiquinone radical, decreased steeply with increasing SDS concentration, reached a minimum value at about 50 mM, and increased gradually with a further increase of concentration. The decrease in k(ESC) in the low-concentration region has been attributed to the decrease in the rate of monomer exchange between the micelle and the bulk phase. On the other hand, the gradual increase in k(ESC) in the high-concentration region has been assigned to the fusion/fission process of the micelle.