It has been believed that photocatalytic oxidation in water proceeds with the reaction of OH radicals generated on the photocatalysts. To explore the actual contribution of OH radicals to photocatalytic oxidation, OH radicals were detected by fluorescence probe method in photoelectrolysis with rutile TiO₂ of (100) and (110) facets. The effect of hydrogen peroxide on the OH radical formation at pH 6.7 was investigated to confirm the relevant intermediate which was suggested in our previous report for water oxidation. In alkaline solutions at pH 9.6 and 12.5, the current efficiencies of OH radical formation were 0.01–0.05%, which are far smaller than those at pH 6.7 (0.2–0.6%) due to the deprotonation of the reaction intermediate as confirmed by FT-IR measurements. These experimental results support a plausible reaction mechanism that the surface Ti–O–O–Ti structure is an intermediate of the water oxidation process, by which mechanism the O₂ production becomes favorable in alkaline solution.