The equilibrium structures, harmonic vibrational frequencies of HONO, and protonated HONO have been investigated using ab initio methods. The ab initio methods include second-order Moller-Plesset (MP2) perturbation theory, single-and double-quadratic configuration interaction (QCISD) theory, and the QCIDS(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The QCISD(T) equilibrium geometry and vibrational frequencies for nitrous acid are in good agreement with experiment. The lowest energy form of protonated HONO is a complex between water and NO+. The next lowest isomer is 35.1 kcal mol(-1) higher in energy. The ab initio proton affinity (PA) of HONO is predicted to be 191.5 kcal mol(-1) at the QCISD(T)/6-311++G(3df,3pd) level of theory.