A method based on photogeneration of OH radicals in water from TiO2 nanoparticles was developed to study the kinetics of oxidation of organic molecules which were used as biological antioxidants. The kinetics of oxidation of terephthalic acid as a reference probe was monitored by fluorescence measurements of the concentration of its oxidized form, 2-hydroterephthalic acid (lambda(ex) = 315 nm, lambda(em) = 425 nm). The kinetics of oxidation of other antioxidant molecules was then deduced from the radical scavenging competition. The antioxidant properties of normal antioxidants were compared based on this kinetic model. And the antioxidant kinetic decreased in the order: lipoic acid, gallic acid, glutathione, uric acid, vitamin C, vitamin E, trolox and bilirubin.