Gas-phase reactions of N2O with H+, Me+, Ph+, PhCH2+, Tr+ (the tropylium. ion) and PhCO+ were studied by pentaquadrupole mass spectrometry. Collision-induced dissociation (CID) of the product ions establishes that, in the diluted solvent and counterion-free MS environment, gaseous Me+ and Ph+ ions form preferentially Me(Ph)O-N-2(+) (electrophilic attack at oxygen), whereas PhCH2+ forms preferentially PhCH2-N2O+ (electrophilic attack at nitrogen). The nascent phenoxydiazonium. ion PhO-N-2(+) dissociates promptly by N-2 loss to form PhO+ as the observable addition product. The PhCO+ and Tr+ ions are unreactive towards addition to N2O. The CID and ion/molecule chemistry of [N2O + H](+) are in conclusive with regard to connectivity, because the ion is rather resistant towards dissociation and reacts essentially as a proton donor species. Gaseous MeO-N-2(+) is not only efficient as a methylating agent towards ethers, heteroaromatics and nitriles, but also displays a rich chemistry that includes polar [4+2(+)] stepwise cycloadditions with representative dienes and polar transacetalization with cyclic acetals. Relative energies and geometries of various RO-N-2(+)/R-N2O+ isomeric pairs were evaluated by MP2 and DFT calculations. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).17077