The biogeochemical cycling of nitrous oxide plays an important role in greenhouse forcing and ozone regulation. Laboratory studies of N2O:N2 mixtures irradiated between 193--207 nm reveal a significant enrichment of the residual heavy nitrous oxide isotopomers. The isotopic signatures resulting from photolysis are well modeled by an irreversible Rayleigh distillation process, with large enrichment factors of &egr;15.18(193 nm)=-18.4,-14.5 per mil and &egr;15.18(207 nm)=-48.7,-46.0 per mil. These results, when combined with diffusive mixing processes, have the potential to explain the stratospheric enrichments previously observed. ¿ 1998 American Geophysical Union