This research article published by Rwanda Journal, 2012Theoretical study of properties of Cs2Cl+, CsCl2−, Cs3Cl2+ and Cs2Cl3− ions has been performed using the density functional theory (DFT/B3LYP) and the second order Møller-Plesset perturbation theory (MP2). Two triple-zeta basis sets, the midsized B1 and the extended one B2, have been used. Equilibrium structure, vibrational spectra, energies and enthalpies of dissociation reactions of the ions have been found. The effect of the basis set size and the computation method on the results has been analyzed. The MP2 method with the extended basis B2 set was accepted as the more accurate among the others. For the triatomic ions Cs2Cl+ and CsCl2−, the equilibrium structure has been found to be linear. For the pentaatomic ions, three isomeric forms were considered. The most abundant isomer in saturated vapor was found to be the angular V-shaped of C2v symmetry for Cs3Cl2+ and the linear of D∞h symmetry for Cs2Cl3−. Thermodynamic functions of the ions were computed and applied to the treatment of experimental mass spectrometric data. Enthalpies of formation ΔfHº(0 K) were determined (in kJ/mol): 51 ± 4 (Cs2Cl+), −622 ± 3 (CsCl2−), −301 ± 8 (Cs3Cl2+), −975 ± 6(Cs2Cl3−).