Analysis of the complete oxygen isotopic composition (16O, 17O, 18O) of tropospheric N2O from various northern hemispheric locations reveals a mass independent anomaly with a 17O excess of Δ17O=1.0¿0.2? at Δ18O=20.7¿0.3?. So far, the origin of this intriguing isotope signature has remained elusive. New laboratory experiments demonstrate that the fractionation during UV photolysis of N2O, which causes 15N and 18O enrichments in the stratosphere, is strictly mass dependent (Δ17O=0). To explain the isotope anomaly in atmospheric N2O, we propose a chemical mechanism for heavy oxygen transfer from O3 to N2O. In a first step, the NOx-O3 photochemical interaction leads to the formation of NO2 with significant excess 17O. In a second step, the heavy oxygen anomaly is transferred to N2O via the reaction NO2+NH2→N2O+H2O, as part of the gas phase degradation of ammonia. This small but significant N2O source is of the right magnitude to explain the tropospheric observations. ¿ 2001 American Geophysical Union