Abstract: A MgCl2.mEtOH.nH2O adducts, where 3.4<m≦4.4, 0≦n≦0.7, characterized by an X-ray diffraction spectrum, taken under the condition set forth above, in which, in the range of 2&thgr; diffraction angles between 5° and 10°, at least two diffraction lines are present at diffraction angles 2&thgr; of 9.3±0.2°, and 9.9±0.2°, the most intense diffraction lines being the one at 2&thgr; of 9.3±0.2°, the intensity of the other diffraction line being less than 0.4 times the intensity of the most intense diffraction line. Catalyst components obtained from the adducts of the present invention are capable to give catalysts for the polymerization of olefins characterized by enhanced activity and/or porosity with respect to the catalysts prepared from the adducts of the prior art.