Hydroxyl radicals ((·)OH) have been deemed to be the major active species during the photocatalytic oxidation reaction. In this study, (·)OH produced on various semiconductor photocatalysts in aqueous solution under Xenon lamp irradiation was quantitatively investigated by the photoluminescence (PL) technique using coumarin (COU) as a probe molecule. The results indicated that the formation rate of (·)OH on the surface of irradiated commercial Degussa P25 (P25) was much higher than that of other semiconductor. The pH values of the solution and phase structure of TiO(2) significantly influenced the production rate of (·)OH. The acidic pH environment of the solutions and bi-phase structure (anatase and rutile) of TiO(2) were beneficial to enhancing the formation rate of (·)OH. In addition, the formation rate of (·)OH on anatase TiO(2) and P25 was much faster than that of (·)OH on the other semiconductors (such as rutile TiO(2), ZnO, WO(3), CdS, Bi(2)WO(4) and BiOCl, etc.). A new concept "OH-index" was first proposed to compare photocatalytic activity of photocatalysts, which would provide new insight into the investigation of semiconductor photocatalysts.