Catalyst components for the polymerization of olefins comprising a titanium halide or alkoxyhalide and an electron-donor compound selected from ethers having two or more ether groups and containing at least one heteroatom selected from the group consisting of N, S, P, Si, non-ether O and halogen atoms or at least one double bond, or both at least one heteroatom and at least one double bond, supported on a magnesium halide in active form.Catalysts obtained from said catalyst components and an Al-alkyl compound, as well as catalysts obtained by the reaction of an Al-alkyl compound and an ether having the above defined characteristics with a solid catalyst component comprising a titanium halide or alkoxyhalide and an electron-donor compound having particular characteristics of extractability with Al-triethyl, supported on activated magnesium halide.