A series of magnesium fluorides (MgF(n)(2-n)), multiply charged anions, in the gas phase and in aqueous solution were theoretically studied with a hybrid approach of quantum chemistry and statistical mechanics, called RISM-SCF-SEDD theory. In the gas phase, MgF(3)(-) is the most stable species among the complexes (n = 1-6). In contrast, due to compensation between the intramolecular energy and solvation free energy, the stabilities of a number of complexes with different n are comparable in aqueous solution. Based on accurate evaluation of free energy change, the mole fraction of MgF(4)(2-) is the highest in the range from pF = 2.0 to 3.0 of aqueous solution. This is consistent with the available PDB data of the enzymes that catalyze the phosphoryl transfer reactions. The hydration structures of magnesium fluorides obtained by RISM-SCF-SEDD theory provide insight into their structural changes from the gas phase to aqueous solution.