A hitherto unknown reaction, the isotope exchange between oxygen and ozone, accompanies formation of the latter upon ionization of O-2 or air, a long-known process widely utilized for the production of ozone and of great relevance to atmospheric chemistry. The isotope exchange was demonstrated and its mechanism established by mass spectrometric techniques that allowed identification of the charged intermediate, the O-5(+) ion, characterized by the joint application of theoretical and experimental methods as truly unique among all known O-n(+) complexes. The results are of interest to atmospheric chemistry, bearing on the problem related to the widely different extent of O-18 enrichment of ozone in the stratosphere and in the troposphere.