In the photochemical reaction of hydrogen peroxide with dimethyl sulfoxide, three spin-polarized transient radical species have been identified by time-resolved EPR techniques: (CH3)(2)SO(OH)(.) (I),(CH3)-C-. (II), and (CH2SOCH3)-C-. (III). The assignments are based on the observed spectral parameters, the deuteration effect, and the time profile analyses. However, the identification of adduct radical (I) is somewhat tentative. To explain the nonrelaxation-limited polarization observed in (CH3)-C-. radicals, a cyclic pathway to continuously regenerate (CH3)-C-. is proposed and verified experimentally. The time evolution of the polarized spin adducts further allows one to follow the entire course of the reaction pathway, yielding important details on the reaction mechanism.