The isotopic analysis of atmospheric nitrous oxide (N2O) has become a valuable tool in the investigation of its sources, sinks, and its atmospheric cycle. In particular the considerable isotopic enrichment accompanying stratospheric photolysis of N2O, its dominant atmospheric sink process, provides a key isotope signal in the construction of a global N2O isotope budget. Here we present the first measurements of the individual fractionation constants for 15N14NO, 15ϵ1=10.9¿1.7? and 14N15NO, 15ϵ2=35.7¿0.5? during ultraviolet photolysis at 193 nm, along with the 18O fractionation constant, 18ϵ=17.3¿0.5?. Consistent results were obtained over a wide range of experimental conditions. The observed position-dependent 15N fractionation confirms theoretical predictions and provides a unique signature of N2O that has been processed in the stratosphere, adding a new dimension to an isotope-based description of the atmospheric N2O budget. ¿ 2000 American Geophysical Union