Abstract: A method of making insulated wires or cables in which a tube of crystallizable polymeric material is first extruded over size on the wire, cooled until it can be gripped and then reheated and stretched to produce crystalline orientation and to collapse it into contact with the wire is distinguished by first holding the tube at a temperature between its glass transition temperature and its crystalline melting point long enough for the degree of crystallinity to be significantly increased. Physical properties after orientation are considerably improved.