The ground potential energy surface of the CO2+·Ar complex has been investigated by ab initio techniques for C∞ν, C2ν, and Cs structures of the ion. The C∞ν and C2ν stationary points are found to be transition states which lead smoothly to the global Cs minimum-energy structure. In all three geometries, the interaction between CO2+ and Ar is found to be largely electrostatic in nature. The equilibrium geometry of the the CO2+·Ar ion is planar with a near linear OCO moiety and a C-O-Ar angle near 100°. At the QCISD(T)/ aug-cc-pVTZ level of theory, De is calculated to be 0.220 eV for dissociation into CO2+ and Ar. Harmonic vibrational frequencies, infrared intensities, and frequency shifts occurring for 13C and 18O calculated at the QCISD/cc-pVDZ (for C and O) and aug-cc-pVDZ (for Ar) level of theory are reported.