Catalyst components for the polymerization of olefins CH2═CHR wherein R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms, comprising Mg, Ti, Cl and a compound (L) or its derivatives, selected from condensed cyclic structures which are formed by at least an aromatic ring and which are substituted with at least two hydroxy groups, said Cl and Ti atoms being in an amount such as to have a molar ratio ranging from 5 to 50.