The Ar-NO+ ionic complex is studied using ab initio calculations. The complex geometry and harmonic vibrational frequencies are calculated using second order Moller-Plesset perturbation theory (MP2) calculations, employing a variety of basis sets. The calculated intermolecular bond length supports the experimental value, whereas the calculated Ar-N-O bond angle suggests a possible reinterpretation of the experimental result. The vibrational frequencies are then recalculated using an anharmonic approach and the fundamentals are found to be in much better agreement with the experimental values [obtained from zero-kinetic-energy (ZEKE) spectroscopy] than are the harmonic values. However, the calculations suggest that the potential energy surface of this complex cation is very anharmonic, and that the experimental assignment of the vibrational features in the ZEKE spectrum may have to be revised. The interaction energy of the complex is calculated, both with and without the full counterpoise (CP) correction; the CP-corrected values are in much closer agreement with experiment than are the uncorrected values. The final value of the stabilization energy, taking into account the MP4 correction is ca. 950 cm-1, in excellent agreement with the (re-evaluated) experimental value of 920+/-20 cm-1.