An alternated copolymer with the formula alpha-hydro-omega-oligo(oxyethylene)hydroxy-poly[oligo(oxyethylene)oxydimethylsililene] and a molecular weight of 9860 was synthesized. Doping of this polymer with the anhydrous salt delta-MgCl2 resulted in a new magnesium electrolytic complex poly[PEG400-alt-DEOS]/(MgCl2)(0.26). The structural hypothesis for the polymer was proposed on the basis of elemental analyses and molecular weight. Detailed H-1-, C-13, and Si-29-NMR spectral investigations fully confirmed the structure of poly[PEG400-alt-DEOS]. Mid-infrared region (MIR) FT-IR studies of the polymer showed that it presents (1) a sufficient number of terminal hydroxyl groups to confer a substantial degree of dissolution and salt-dissociation in the polymer complex; and (2) polyethereal fragments in a conformational geometry close to TGT (T = trans, G = gauche). The conductivity against temperature plot for this very amorphous magnesium polymer electrolyte demonstrated that the material conducts ionically by means of two types of charge migration mechanisms.