The addition of NO2+ to ethylene, the prototypal electrophilic nitration of a pi system and the focus of considerable theoretical interest as a model of aromatic nitration, has been studied in the gas phase by FT-ICR, MIKE, and CAD mass spectrometry, complemented by ab initio calculations at the MP2/6-31+G* level of theory. The results provide a clearcut answer to the principal mechanistic question addressed, showing that the reaction yields a O-nitroso product, probably CH3CHONO+, rather than a C-nitrated product.