The gas phase and aqueous thermochemistry, NMR chemical shifts, and the topology of chemical bonding of nitrous acid (HONO) and nitrous acidium ion (H2ONO+) have been investigated by ab initio methods using density functional theory. By the same methods, the dissociation of H2ONO+ to give the nitrosonium ion (NO) and water has also been investigated. We have used Becke's hybrid functional (B3LYP), and geometry optimizations were performed with the 6-311++G(d,p) basis set In addition, highly accurate ab initio composite methods (G3 and CBS-Q) were used. Solvation energies were calculated using the conductor-like polarizable continuum model, CPCM, at the B3LYP/6-311++G(d,p) level of theory, with the UAKS cavity model. The pK(a) value of H2ONO+ was calculated using two different schemes: the direct method and the proton exchange method. The calculated pKa values at different levels of theory range from -9.4 to -15.6, showing that H2ONO+ is a strong acid (i.e., HONO is only a weak base). The equilibrium constant, K-R, for protonation of nitrous acid followed by dissociation to give NO and H2O has also been calculated using the same methodologies. The pK(R) value calculated by the G3 and CBS-QB3 methods is in best (and satisfactory) agreement with experimental results, which allows us to narrow down the likely value of the pKa of H2ONO+ to about -10, a value appreciably more acidic than literature values.