Abstract: A process for chlorinating vinyl polymers, especially vinyl chloride polymers (i.e. homopolymers and copolymers having a chlorine content of less than about 56.5% by weight) to increase the chlorine content (to a level preferably in excess of 60% by weight), wherein the vinyl chloride polymer is mixed in a powdered state with a minor proportion (in terms of the total mixture) of at least one chloroalkane (preferably 10 to 55 parts by volume in cm.sup.3 per 100 parts by weight in grams of polymer and more generally 10 parts by weight to 85 parts by weight of the chloroalkane per 100 parts by weight of the polymer), and then chlorinating the mixture with molecular chlorine (Cl.sub.2) at a temperature below the vetrification or vitreous-transistion temperature (second-order glass-transition temperature) of the polymer.