Display omitted •Photolysis of oxime ester compound was studied by time-resolved and pulsed EPR.•Spin polarized Iminyl and benzoyloxy radicals were initially generated.•Decarboxylation rate of benzoyloxy in various solvents were measured by pulsed EPR.•Dehydrogenated solvent radicals were observed after the photolysis.•Addition rate of dehydrogenated cyclohexane radical with olefins were determined. To generate spin-polarized free radicals, the photolysis of 1,2-octane-dione, 1-4-(phenylthio)-2-(o-benzoyloxime) (OXE01) was investigated using time-resolved electron paramagnetic resonance (TR-EPR) and pulsed EPR. This photolysis proceeds through α-cleavage of the N-O bond of OXE01 to yield iminyl and benzoyloxy radicals, the latter of which is converted to a phenyl radical by decarboxylation. The phenyl radical promptly abstracts hydrogen atoms from the surrounding solvent molecules to produce a solvent dehydrogenated radical (solvent radical). The spin polarization created in the initial OXE01 triplet excited state was preserved during these series of reactions, which enhanced EPR sensitivity. This enabled us to observe the TR-EPR spectra of the intermediate free radicals, except phenyl. The decarboxylation rate of the benzoyloxy radical was measured in various solvents by probing the free induction decay signal of the product solvent radicals using pulsed EPR. This new method for preparing free radicals with spin-polarization was applied to determine the addition reaction rate constants of the cyclohexyl radical, a solvent radical, to olefins using pulsed EPR monitoring of the electron spin echo signals of the radical.