Electrolyte solutions for rechargeable Mg batteries were developed, based on reaction products of phenyl magnesium chloride (PhMgCl) Lewis base and Al Cl3 Lewis acid in ethers. The transmetallation of these ligands forms solutions with Mgx Cly+ and Al Cl4-n Phn- ions as the major ionic species, as analyzed by multinuclei nuclear magnetic resonance spectroscopy. Tetrahydrofuran (THF) solutions of (PhMgCl)2 -Al Cl3 exhibit optimal properties: highly reversible Mg deposition (100% cycling efficiency) with low overvoltage: <0.2 V and electrochemical windows wider than 3 V. A specific conductivity of 2-5× 10-3 -1 cm-1 could be measured between -10 and 30°C for these solutions, similar to that of standard electrolyte solutions for Li batteries. Mg ions intercalate reversibly with Chevrel phase (Mgx Mo6 S8) cathodes in these solutions. These systems exhibit high thermal stability. The solutions may enable the use of high voltage, high-capacity Mg insertion materials as cathodes and hence open the door for research and development of high-energy density, rechargeable Mg batteries.