Photolysis of (17,18)O-labeled water in the presence of molecular oxygen is accompanied by transfer Of (17)O and (18)O isotopes from water to oxygen, demonstrating. that photoinduced oxidation of water does occur. The reaction exhibits the following isotope effect : oxidation of H(2)(17)Ois faster by 26% (in the Earth's magnetic field) and by 6.0% (in the field 0.5 T) than that of H(2)(18)O. The effect is supposed to arise in the two spin-selective, isotope-sorting reactions recombination and disproportionation-in the pairs of encountering HO(2) radicals. The former is spin allowed from the singlet state; the latter occurs only in the triplet one. Nuclear spin sorting produced by these reactions proceeds in opposite directions with the dominating contribution of recombination, which provides observable (17)O/(18)O isotope fractionation in favor of magnetic isotope (17)O. Neither isotope exchange nor the reaction itself Occurs in the dark.