The strong fluorescence of 7-hydroxycoumarin yielded by the reaction of coumarin with hydroxyl radicals (?OH) was used as a fluorescent probe to determine the yield of 7-hydroxycoumarin aqueous solutions with different concentrations of TBP and TiAP after gamma ray irradiation. The reaction rate constants of TBP and TiAP with ?OH at room temperature were determined via pseudo-first order kinetic fitting ($k_{\mathrm{T}\mathrm{B}\mathrm{P}}$= (9.0 ± 0.2) ×10⁹ L/（mol·s）and$k_{\mathrm{T}\mathrm{i}\mathrm{A}\mathrm{P}}$= (5.3 ± 0.2) ×10⁹ L/（mol·s）),respectively. Owing to the stronger hydrophobicity of TiAP compared with that of TBP,its reaction rate constant with ?OH is lower than that of TBP,thus resulting in a higher yield of ?OH in the TiAP solution compared to the TBP solution with the same concentration. This study provides a simple,convenient and efficient method for investigating the reactions between extractants and ?OH.