The equilibrium structures, dipole moments, harmonic vibrational frequencies, and infrared intensities of methyl nitrate, methanol, and ten structures of protonated methyl nitrate have been investigated using state-of-the-art ab initio quantum mechanical methods. The ab initio methods include self-consistent field (SCF), second-order Møller-Plesset (MP2) perturbation theory, single-and double-excitation coupled-cluster (CCSD) theory, and the CCSD(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The MP2 equilibrium geometry and vibrational frequencies of methyl nitrate and methanol are in good agreement with experiment. The lowest energy gas-phase form of protonated methyl nitrate is a complex between methanol and NO2+, although the next lowest isomer is only 4.9 kcal/mol higher in energy. The CH3OH···NO2+ complex is bound by 19.6 ± 2 kcal/mol. The ab initio proton affinity (PA) of methyl nitrate is 176.9 ± 5 kcal/mol, in very good agreement with the experimental value of 176 kcal/mol. The results of this study are contrasted with an earlier study on protonated nitric acid, and it is shown that methyl nitrate possesses a smaller PA than nitric acid. An explanation for this phenomenon is presented. © 1992, American Chemical Society. All rights reserved.