Benzophenone ((C₆H₅)₂CO) and decafluorobenzophenone ((C₆F₅)₂CO) were applied to elucidate the photochemical reaction pathway of hydrogen peroxide (H₂O₂) with dimethylsulfoxide (DMSO). When a solution of benzophenone in DMSO was excited with the 355 nm laser light, three transient species were observed in the time-resolved electron paramagnetic resonance spectra: benzophenone ketyl (C₆H₅)₂COH^⋅, methyl^⋅CH₃, and methylsulfinic methyl^⋅CH₂SOCH₃ radicals. However, when decafluoro-benzophenone was used with DMSO, only ketyl and methylsulfinic methyl radicals were observed under the same experimental conditions. When the reaction of benzophenone and DMSO was carried out in the presence of H₂O₂, different time profiles of^⋅CH₃ radicals were observed. In the reaction of decafluorobenzophenone-DMSO-H₂O₂, the time profiles of the radicals were not affected by the presence of H₂O₂. Thus, these results verify that^⋅CH₃ radicals are regenerated in a cyclic pathway, in which^⋅CH₃ radicals attack H₂O₂. The regeneration pathway allows us to observe f-pair polarization throughout the lifetime of^⋅CH₃ radicals, which last several microseconds, an order of magnitude longer than theT ₁ relaxation time of^⋅CH₃ radicals.