This research article published by Springer Nature Switzerland AG., 2011The geometrical structure, the frequencies of normal vibrations, and the thermodynamic characteristics of tri- and pentaatomic cluster ions M2Cl+, MCl 2 − , M3Cl 2 + , and M2Cl 3 − (M = K, Rb, or Cs) existing in saturated vapors over potassium, rubidium, and cesium chlorides have been calculated by nonempirical methods. According to the calculations, the triatomic ions have a linear equilibrium configuration of D ∞h symmetry. For the pentaatomic ions M3Cl 2 + and M2Cl 3 − , three isomeric forms were considered. The most abundant isomer in saturated vapor was found to be the linear one (D ∞h) for K3Cl 2 + , Rb3Cl 2 + , K2Cl 3 − , Rb2Cl 3 − and Cs2Cl 3 − , and the angular V-shaped (C 2v) for Cs3Cl 2 + . The concentration of the other two isomers, cyclic and bipyramidal, is negligible. The energies and enthalpies of dissociation of the ions have been found. The enthalpies of formation of the ions, Δf H°(0 K) in kJ/mol, were obtained: 116 ± 3 (K2Cl+), −623 ± 3 (KCl 2 − ), −228 ± 7 (K3Cl 2 + ), −971 ± 6 (K2Cl 3 − ), 91 ± 3 (Rb2Cl+), −620 ± 3 (RbCl 2 − ), −256 ± 5 (Rb3Cl 2 + ), −967 ± 4 (Rb2Cl 3 − ), 51 ± 4 (Cs2Cl+), −622 ± 3 (CsCl 2 − ), −301 ± 8 (Cs3Cl 2 + ), −975 ± 6 (Cs2Cl 3 − ).