Catalyst components for the polymerization of olefins CH.sub.2.dbd.CHR, wherein R is hydrogen or a hydrocarbon radical having 1 12 carbon atoms, comprising Mg, Ti, Cl, OR groups, where R is a C1 C10 alkyl group optionally containing heteroatoms, and an ether having two or more ether groups, characterized by the fact that the Mg/Ti weight ratio is lower than 3 from 2 to 6.5 the Cl/Ti weight ratio is from 1.5 to 6, the OR/Ti weight ratio is from 0.5 to 3.5 and at least 50% of the Titanium atoms are in a valence state lower than 4. The said catalyst components allow the preparation of ethylene copolymers with a low content of xylene soluble fractions.