Photodissociation of CO2 by ultraviolet light (&lgr;=185 nm) generates CO and O2 which are unusually enriched (more than 100?) in 17O. The dissociation takes place through a spin forbidden process during transition from a singlet to a triplet state, the latter lying on a repulsive potential energy surface. The 17O isotopic enrichment is a primary process associated with this transition and could be due to near resonant spin-orbit coupling of the low energy vibrational levels of the 16O12C17O molecule in the singlet state with those of the triplet state near the zone of transition. In contrast, photodissociation at shorter wavelengths (&lgr;<160 nm) involves no spin violation and produces CO and O2 which are fractionated in a conventional mass dependent fashion. The proposed explanation is further supported using 13C enriched CO2; in this case the products are enriched in both heavy isotopes but about 100? more in 18O. The 17O enrichment in CO and O2 generated by CO2 photolysis in a range of UV wavelengths may be a useful tracer in delineating processes in the atmospheres of Earth and Mars. ¿ 2000 American Geophysical Union