The C2H4NO+ system has been examined by means of quantum chemical calculations using the G2 and G3B3 approaches and tandem mass spectrometry experiments. Theoretical investigation of the C2H4NO+ potential-energy surface includes 19 stable C2H4NO+ structures and a large set of their possible interconnections. These computations provide insights for the understanding of the (i) addition of the nitrosonium cation NO+ to the ethylene molecule, (ii) skeletal rearrangements evidenced in previous experimental studies on comparable systems, and (iii) experimental identification of new C2H4NO+ structures. It is predicted from computation that gas-phase nitrosation of ethylene may produce C2H4 center dot NO+ adducts, the most stable structure of which is a pi-complex, 1, stabilized by ca. 65 kJ/mol with respect to its separated components. This complex was produced in the gas phase by a transnitrosation process involving as reactant a complex between water and NO+ (H2O center dot NO+) and the ethylene molecule and fully characterized by collisional experiments. Among the other C2H4NO+ structures predicted by theory to be protected against dissociation or isomerization by significant energy barriers, five were also experimentally identified. These finding include structures CH3CHNO+ (5), CH3CNOH+ (8), CH3NHCO+ (18), CH3NCOH+ (19), and an ion/neutral complex CH2O center dot center dot center dot HCNH+ (12).