This research article published by the British Journal of Applied Science & Technology, Vol.: 9, Issue.: 2, 2015The properties of ions Cs2X+, Cs3X2+, CsX2−, and Cs2X3− (X = Br or I) have been studied using the density functional theory and Möller–Plesset perturbation theory of the 2nd and 4th order. For all species the equilibrium geometrical configurations and vibration frequencies were determined. Different isomers of pentaatomic ions were found to exist: the linear (D∞h), V-shaped (C2v), kite-shaped (C2v) and bipyramidal (D3h). The relative abundances of isomers were calculated for temperatures between 700 K and 1600 K. It was found that at about 800 K, the amount of different isomers was comparable for Cs3Br2+, Cs3I2+ and Cs2I3− ions, while for Cs2Br3− the linear isomer was proved to be predominant. The enthalpies of dissociation reactions with elimination of CsX molecules and the enthalpies of formation of ions were determined.