The position dependent "N fractionation of nitrous oxide (N20). which cannot be obtained from mass spectrometric analysis on molecular N20 itself, can be determined with high precision using isotope ratio mass spectrometry on the NO+ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent 15N fractionations occur in the photolysis of N2O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N20 indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for 15N'4N0 and ''N'5N0. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dyaamicai and also additional chemical processes like the reaction of N20 with WID).