Silicon oxidation by oxygen depends on the magnetic field and exhibits isotope effect: silicon atoms 29Si with magnetic nuclei are oxidized twice as fast as atoms 28Si and 30Si with nonmagnetic nuclei. Both effects certify that the oxidation is a spin selective reaction in which radical pairs participate. The dominating oxidizing species are oxygen molecules in triplet spin state which generate triplet radical pairs. Zeeman interaction and hyperfine coupling with 29Si nuclei in these pairs stimulate triplet–singlet spin conversion and transform triplet pairs into the reactive singlet ones. The subsequent spin allowed reactions in the latter result in the oxidation products.