The halogen and organic substances content of the catalyst residues in alpha-olefin crystalline (co)-polymers prepared with catalysts obtained by reacting:(a) a metallorganic Al compound with(b) a Ti compound supported on a Mg halide, in which an electron-donor is combined with the Ti compound and/or the Mg compound and optionally with (a),are reduced by passing a stream of superheated steam (optionally mixed with other gaseous fluids) over the polymer in powder form at a temperature of 105.degree.-140.degree. C. and under a pressure of 0.1-10 kg/cm.sup.2 gauge, the steam/polymer ratio being comprised between 0.10 and 1 by weight.