By X-ray diffraction on polycrystalline powders and Fourier-transform IR spectroscopy, some adducts MgCl2(C2H5OH)x were studied, having different composition in the range from x = 0 (α-MgCl2) to x = 6. For 2 < x ≤ 6, the structure of such adducts shows prevalently ionic Mg-Cl bonds. For x ≤ 2, covalent bonds are observed among chlorine and magnesium atoms, which give rise to chain compounds through chlorine bridges among the Mg-atoms. The obtained structural data are also discussed in terms of the synthesis of some MgCl2-based supports which are very active in the Ziegler-Natta process of olefin polymerization, after a treatment with titanium halides and a successive addition of a cocatalyst and of other suitable donors.