Abstract: A simple and practical method for making post-chlorinated polymers based upon vinyl chloride is provided based upon conducting the chlorination reaction on a granular polymer suspended in aqueous hydrochloric acid in the presence of a stoichiometric excess of liquid chlorine and in the absence of any significant amount or organic solvent/swelling agent. Use of photo-radiation techniques is also avoided since the reaction is activated by means of a minor fraction of a percent based upon weight of said polymer of an organic peroxy compound which decomposes to form free radicals at the temperatures involved in the reaction.