The effect of MgCl2 on a series of chloride containing magnesium electrolytes was investigated. In the presence of extra MgCl2, the electrochemical properties of Grignard reagents (RMgCl, R = Ph, Et, iPr) were significantly improved, and the advantage of MgCl2 was further demonstrated in Mg-Mo6S8 rechargeable batteries with improved capacities and much smaller over-potentials. MgCl2 was then further proven to be a powerful reagent to improve the performance of well-established strong Lewis acid derived magnesium electrolytes including the "all-phenyl" complex (APC) and alkoxide-based magnesium electrolytes. The results suggest that MgCl2 salt is a very important species to benefit all chloride containing electrolytes for rechargeable magnesium-ion batteries. (C) 2016 The Electrochemical Society. All rights reserved.