The reaction of singlet oxygen (¹O₂) generated by ultraviolet-A (UVA)–visible light (λ > 330 nm) irradiation of air-saturated solutions of hematoporphyrin with phenolic compounds in the presence of a spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), gave an electron spin resonance (ESR) spectrum characteristic of the DMPO–hydroxyl radical spin adduct (DMPO–^·OH). In contrast, the ESR signal of 5,5-dimethyl-2-pyrrolidone-N-oxyl, an oxidative product of DMPO, was observed in the absence of phenolic compounds. The ESR signal of DMPO–^·OH decreased in the presence of either a ^·OH scavenger or a quencher of ¹O₂ and under anaerobic conditions, whereas it increased depending on the concentration of DMPO. These results indicate both ¹O₂- and DMPO-mediated formation of free ^·OH during the reaction. When DMPO was replaced with 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DEPMPO), no DEPMPO adduct of oxygen radical species was obtained. This suggests that ¹O₂, as an oxidizing agent, reacts little with DEPMPO, in which a strong electron-withdrawing phosphoryl group increases the oxidation potential of DEPMPO compared with DMPO. A linear correlation between the amounts of DMPO–^·OH generated and the oxidation potentials of phenolic compounds was observed, suggesting that the electron-donating properties of phenolic compounds contribute to the appearance of ^·OH. These observations indicate that ¹O₂ reacts first with DMPO, and the resulting DMPO–¹O₂ intermediate is immediately decomposed/reduced to give ^·OH. Phenolic compounds would participate in this reaction as electron donors but would not contribute to the direct conversion of ¹O₂ to ^·OH. Furthermore, DEPMPO did not cause the spin-trapping agent–mediated generation of ^·OH like DMPO did.