Photolysis of 17,18O-labeled water in the presence of molecular oxygen is accompanied by transfer of 17O and 18O isotopes from water to oxygen, demonstrating that photoinduced oxidation of water does occur. The reaction exhibits the following isotope effect: oxidation of H217O is faster by 2.6% (in the Earth's magnetic field) and by 6.0% (in the field 0.5 T) than that of H218O. The effect is supposed to arise in the two spin-selective, isotope-sorting reactions―recombination and disproportionation―in the pairs of encountering HO2 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 17O/18O isotope fractionation in favor of magnetic isotope 17O. Neither isotope exchange nor the reaction itself occurs in the dark.