The iso-nitrous acid radical-cation, HNO2+., was prepared for the first time by exothermic protonation of NO2 with H-3(+) and characterized by collisionally activated dissociation mass spectra. RRKM calculations that were based on B3LYP/6-311 + + G(3df,2pd) calculated energies and frequencies were consistent with experimental data that showed the loss of H-. and HO. radicals to be the only two competitive fragmentation channels of HNO2+.. (C) 2004 Elsevier B.V. All rights reserved.