Ab initio molecular orbital calculations have been performed on the NO2+. X complexes (X=H2O, N-2, and CO2). Optimized geometries and vibrational frequencies, as well as computed total energies have been used to calculate standard enthalpies, entropies, and free energies for the complexing reactions, and ligand-switching reactions between the three molecular complexes. The results obtained have been compared with experimental values where available. The following thermodynamic values are recommended for the association reaction NO2++X-->NO2+. X : Delta H-298 = -16.8, -4.7, and -7.9 kcal mol(-1) and Delta S-298 = -23.1, -20.2, and -20.8 cal mol(-1) K-1, for X=H2O, N-2, CO2, respectively. Corresponding thermodynamic values for the ligand-switching reactions have also been calculated. Minimum energy geometries calculated for NO2+. H2O and NO2+. N-2 are found to be in agreement with those obtained in previous theoretical studies. The NO2+. CO2 complex has been studied for the first time, and the minimum energy geometry has been shown to be planar and of C-2v geometry.