Abstract The oxygen gases in a fused silica container cooled to 77K at pressures of 0.5 to 15 cm Hg were energized by an RF generator of 60 kHz to produce ozone which condensed on the cold wall. The isotopomeric compositions of the initial O 2, the O 2 remaining after the discharge, and the O 2 which was completely converted from the O 3 were measured. Oxygen gases of three different isotopic compositions were used. In addition, oxygen pressure, degree of ozone formation, and geometry of the discharge were varied. The isotopomeric data of O 2 gases after the discharge reactions showed very unusual patterns. These patterns depend mainly on the isotopic composition of the oxygen used for the reaction. We compared the data with a model based on the effect of the anharmonicity and the symmetry of the excited ozone molecules on their predissociation. The anharmonicity and symmetry effects depend on the isotopic composition of the O 2 which in turn control the isotope fractionation associated with the formation of ozone. Our data are not compatible with several models such as self-shielding and the structural symmetry of the initial oxygen gas.