The structures, frequencies, and stabilization energies of the magnesium dihalide dimers (MgX(2))(2) (X = F, Cl, Br, and I) and trimers (MgX(2))(3) (X = F, Cl) are investigated using ah initio molecular orbital techniques. Double-bridged (D-2k) and triple-bridged (C-3v) dimers and bridged (D-2d) trimers are found to be local minima on the potential energy surfaces for all the magnesium dihalides, with the double-bridged dimers being consistently lower in energy than the corresponding triple-bridged dimers. The exothermicities of the dimerization reactions, 2MgX(2) --> (MgX(2))(2) (D-2h Or C-3v), and the trimerization reactions, 3MgX(2) --> (MgX(2))(3) (D-2d), generally decrease as the size of the halide increases. The computed frequencies of the dimers support the experimental spectral identification of these structures in magnesium dihalide vapors reported by Lesiecki and Nibler (J. Chem. Phys. 1976, 64, 871).