A process for the (co)polymerization ethylene, optionally in mixtures with olefins CHCHR in which R is hydrogen or a hydrocarbyl radical with 1-12 carbon atoms, carried out in the presence of a catalyst system comprising (A) a solid catalyst component which comprises Mg, halogen an electron donor selected form ethers, esters, or amines, and Ti atoms in an oxidation state such that the weight percentage ratio between Ti^(red)/Ti_(tot) ranges from about 0.05 to about 1; wherein Ti^(red) is the weight percentage on the solid catalyst component of the Ti atoms having a valence less than 4 and Ti_(tot) is the weight percentage on the solid catalyst component of all the Ti atoms and (B) an Al-alkyl compound. The said process is capable to produce ethylene polymers with a reduced oligomers content and/or improved mechanical characteristics.