This paper describes ab initio computations of the molecular structures, binding energies, entropies, and specific heats of the complex ion NO+⋅.N2, NO+⋅O2, and NO+⋅CO2. The computed binding energies are 0.23, 0.15, and 0.42 eV, respectively; the standard entropies at 298 K are 74.0, and 79.8, and 77.5 cal/deg mol, and the molar heat capacities Cp(298 K) are 15.6, 15.8, and 17.2 cal/deg mol. For the case of NO+.N2 the computed heat capacity is used in conjunction with previously published equilibrium data to obtain a revised experimentally determined value for the binding energy, namely, 0.21 eV, in fairly good agreement with the computed value. The computed binding energies for the other ions are compared with other previously published estimates. Applications of these results to the chemistry of the ionospheric D region are discussed. ¿American Geophysical Union 1990