A concentration-dependent vibrational spectroscopic study is carried out for the 1:1 MgCl2:AlCl3-DME system and detailed information on the precursor and the electrochemically active species formed in this rechargeable Mg battery electrolyte are presented for the first time in solution. An equilibrium between trans-[MgCl2(DME)2] and cis-[MgCl2(DME)2] isomers is proposed as well as the formation of [Mg2(μ-Cl)2]2+ and [AlCl4]− by using Raman and IR spectroscopies. The combination of the quantitative analysis made at the CO stretching region of DME with the spectral trend exhibited in low wavenumber suggests the existence of the [Mg2(μ-Cl)2(DME)4]2+ complex, similar to structure previously resolved by X-ray diffraction, in which two [AlCl4]− anions maintain the neutrality of the system.