The structures of small magnesium dichloride clusters were studied by ab initio quantum chemical methods. A technique was developed to take all conceivable structural isomers systematically into account. Altogether 168 isomers of (MgCl2)(n), where n = 1-4, were optimized, from which data structural characteristics were determined. Generally, magnesium prefers coordination numbers of 3 and higher. Planar 3-coordinated and tetrahedral 4-coordinated orientations are particularly favored. The most stable isomer of the series of compounds studied, the one possessing tetrahedral 4-coordinated magnesium atoms, resembles a fragment of the magnesium dichloride lattice. On the basis of this tetramer, higher oligomers up to nonamers were studied, demonstrating the convergence of (MgCl2)(n) clusters to the bulk structure of MgCl2.