Nitrous oxide (N2O) extracted from stratospheric whole air samples has been analyzed for its 15N and 18O isotopic composition, and strong enrichments in the heavy isotopes are observed concomitant with decreasing N2O mixing ratio. Notably, the 15N enrichment is strongly different at the two nonequivalent positions in the molecule. Laboratory broadband photolysis experiments at wavelengths representative for the stratosphere confirm that photolysis is the prime cause for the observed fractionation in the stratosphere. However, the in situ stratospheric fractionation constants are significantly reduced compared to the laboratory data, reflecting the importance of dynamic processes. In addition, small but significant variations in the ratio of the two 15N fractionation constants indicate the influence of additional chemical processes like the oxidation of N2O by O(1D). ¿ 2001 American Geophysical Union