Modification of TiO2 nanoparticles with dopamine enables harvesting of visible light and promotes spatial separation of charges. The formation of reactive oxygen species (OH, O-1(2), O-2, HO2, H2O2) upon illumination of TiO2/dopamine was studied using complementary spin-trap EPR and radical-induced fluorescence techniques. The localization of holes on dopamine suppresses oxidation of adsorbed water molecules at the surface of nanoparticles, and thus formation of OH radicals. At the same time, dopamine does not affect electronic properties of photogenerated electrons and their reaction with dissolved oxygen to produce superoxide anions. Superoxide anions are proposed to generate singlet oxygen through dismutation reaction, resulting in a low yield of O-1(2) detected.