Ozonation of nucleic constituents in pure water in semi-batch reactor, showed that nucleobases and nucleosides behave in a similar way, regarding ozone consumption and inorganic nitrogen formation. Stoichiometric ratios were found about 1 mole of consumed ozone per mole of oxidized nucleobase and nucleoside, except for adenine and adenosine which presented a stoichiometric ratio near 3 moles per mole. Concerning inorganic nitrogen formation, we noted that the presence of primary amino groups induced ammonia and nitrate formations, but never more than one mole per mole of initial compound. Carbon dioxide and other end-products were released during ozonation of adenine and thymine. GC/MS analyses showed the presence of some simple end-products, such as oxalic acid, urea and carbamic acid. However trioxotriazine (or cyanuric acid) seemed to be an important end-product, issued from a cyclic trimerization of formamide followed by oxidation of carbon-nitrogen double bonds.