The solid phase-solution equilibrium has been studied in the MgCl2-tetrahydrofurane system. Below 28 °C the saturated solution of magnesium chloride is in equilibrium with the solid phase composed of MgCl2.4C4H8O, whereas at higher temperatures the equilibrium solvate has the composition of MgCl2.2C4H8O. The solvate MgCl2.4C4H8O forms tetragonal crystals composed of molecules of trans-dichloro-tetrakis(tetrahydrofurane)magnesium(II) complex. The solvate MgCl2.2C4H8O most probably represents a polymeric catena-di-μ-dichloro-trans-bis(tetrahydrofurane)magnesium(II) complex.