The Schlenk equilibrium involving RMgX, R2Mg, and MgX2 (R = Me, Et, Ph and X = Cl, Br) has been studied both in the gas phase and in diethyl ether (Et2O) and tetrahydrofuran (THF) solutions by means of the density functional theory (DFT) B3LYP/6-31+G* method. Solvation was modeled using the supermolecule approach. The stabilization due to interaction with solvent molecules decreases in the order MgX2 > RMgX > R2Mg and among the groups (R and X) Ph > Me > Et and Cl > Br. Studied magnesium compounds are more strongly solvated by THF compared to Et2O. The magnesium halide is solvated with up to four solvent molecules in THF solution, assuming that trans-dihalotetrakis(tetrahydrofurano)magnesium(II) complex forms. The formation of cis-dihalotetrakis(tetrahydrofurano)magnesium(II) is energetically less favorable than the formation of corresponding disolvated complexes. The predominant species in the Schlenk equilibrium are RMgX in Et2O and R2Mg + MgX2 in THF, which is consistent with experimental data.