Gaseous [HN2O5](+) ions formed upon addition of NO2+ to nitric acid have been studied by mass spectrometric and computational methods. The results from MIKE, CAD and FT-ICR spectrometry and calculations at the B3LYP 6-311++G(3df, 3dp)//6-311G(d,p) level of theory show that the most stable adduct formed is an electrostatic HNO3 . NO2+ complex where NO2+ is coordinated to the nitro group of nitric acid. Consequently, the nitro group is the energetically preferred protonation site of N2O5, whose experimental proton affinity (PA) amounts to 189.8 +/- 2 kcal mol(-1), vs theoretically computed values ranging from 182 to 188 kcal mol(-1). Addition of NO2+ to XNO2 molecules (X = CH3O, C2H5O and NH2) also yields electrostatic complexes where the nitronium ion is coordinated to the NO2 group. The most stable [HN2O4](+) ion from the addition of NO+ to HNO3 is also identified as a cluster characterized by coordination of the nitrosonium ion to the nitro group, whose almost thermoneutral isomerization into a cluster where a nitronium ion is coordinated to the nitroso group of HNO2 is characterized by a sizable barrier. The larger PA of N2O5 than of H2O and HNO3 is of interest in atmospheric chemistry, pointing to protonation by H3O+ and/or H2NO3+ ions as the first step of the N2O5 destruction in ionic Clusters and aerosols.