The structural and thermodynamic properties of oligomers of Group IIIA metal trihalides [MX₃] _n (M = Al, Ga, In; X = F, Cl, Br, I; n = 2, 3, 4) have been obtained by the density functional theory B3LYP/LANL2DZ(d) method. The processes of tri-and tetramerization of trifluorides are exothermic and favorable at low temperatures. The fraction of trimeric molecules in aluminum and gallium chloride vapors is small (∼10^-3%). For Al₃Cl₉ and Al ₄Cl₁₂, in addition to the expected cyclic structures with three and four chloride bridges, the existence of their high-energy isomers with, respectively, six and eight bridging halogen atoms has been predicted.