Full Text Article. Also available at: http://dx.doi.org/10.1080/23312009.2016.1186582The properties of trimer Cs3X3 and tetramer Cs4X4 (X = Br, I) molecules have been studied using DFT with B3LYP5 functional and MP2 and MP4 methods. Two equilibrium geometrical structures of trimers, hexagonal (D3 h) and “butterflyshaped” (Cs), and one for tetramers, distorted cubic (Td), are confirmed to exist; geometrical parameters and vibrational spectra are determined. The relative concentration of Cs3X3 isomers has been evaluated; the butterfly-shaped isomer dominates over hexagonal in saturated vapour in a broad temperature range. The dissociation reactions through different channels have been considered and enthalpies of formation ΔfH°(0) of clusters determined:−858 } 20 kJ mol−1 (Cs3Br3), −698 } 20 kJ mol−1 (Cs3I3), −1270 } 30 kJ mol−1 (Cs4Br4) and −1045 } 30 kJ mol−1 (Cs4I4). The Gibbs free energies ΔrG°(T) calculated for the dissociation reactions of trimer and tetramer molecules have indicated that these molecules are resistive in narrow temperature range only and decompose spontaneously with temperature increase with elimination of dimer molecules.