Spherical adducts comprising a MgCl2, an alcohol ROH in which R is a C1-C10 hydrocarbon group, present in a molar ratio with MgCl2 ranging from 0.5 to 6 and less than 20% mol based on the mol of MgCl2 of a compound of formula Mg(OR1)2 in which R1 is selected from C1-C10 alkyl groups or R2CO groups in which R2 is selected from C1-C6 alkyl or aryl groups.