The mass independent isotopic fractionation is O3 formation from O2 photolysis has been investigated as a function of pressure (2.1--760 Torr), UV wavelength and differing third body. In the low pressure regime (≲200 Torr room temp., ≲10 Torr liquid nitrogen), O3 decomposition occurs, resulting in a secondary, mass-dependent isotopic fractionation. It is demonstrated that wall effects produce O3 in a mass-independent fashion, thus indicating the involvement of rotational states in the mass independent fractionation process. For pressures in the intermediate range, to ~600 Torr, a single-stage fractionation is observed, presumably with Δ17O≂18O≂85--100%, though secondary effects may be operative also. At pressures above 450 Torr, the Δ18O of O3 decreases to 87% at 760 Torr. The fractionation process appears to be independent of UV wavelength and third body composition; photolysis of air produces O3 isotopically similar to pure O2 photolysis at comparable pressures. ¿ American Geophysical Union 1988