The ratio of the amount of carboxymethyl radicals to that of methyl radicals was measured in situ for the photocatalytic decomposition of acetic acid over platinized TiO2 powder in deoxygenated aqueous solution at room temperature and discussed in connection with the contribution of . OH radicals in the oxidation process. Since carboxymethyl radicals are formed only by indirect oxidation, the radical ratio was used to measure the indirect oxidation via . OH radicals. Thus, ratios of the indirect oxidation were estimated for five commercially available TiO2 photocatalysts, in addition to the several properties of the powders. The ratios were found to be sensitive to the characteristics of the TiO2 catalysts. The larger the amount of surface OH group, the more likely the indirect oxidation via . OH radicals occurs. This could be attributed to the difference in the structures of the surface trapped holes elucidated by the ESR measurements of the corresponding TiO2 powder at 77 K.