Substitution of surface −OH groups of may influence competition between photoinduced energy and electron-transfer processes and lead to improved singlet oxygen generation. In contrast to neat , surface modified titanium dioxide can photocatalyze degradation of a very stable substance, cyanuric acid. Presented results are in contradiction to usually accepted hypothesis of the mechanism of cyanuric acid photodegradation in the presence of fluorinated (F−) in which "bulk" hydroxyl radicals play a key role. It seems plausible that the difference of activity observed for and F− is not related to various types of generated hydroxyl radicals, but rather to competition between electron-transfer processes (formation of radicals) and energy-transfer processes (formation of singlet oxygen).