TRXLPE-type cable sheaths are prepared by a method in which a solid polymer is mixed with a liquid water tree-resistant agent either by dosing or direct injection. In the dosing method, the solid polymer, e.g., high pressure LDPE, is sprayed or otherwise contacted with the liquid agent, e.g., PEG, the agent is allowed to absorb into the polymer, and the polymer with absorbed agent is then fed to an extrusion apparatus for extrusion over a sheathed or unsheathed wire or optic fiber. In the direct injection method, the solid polymer is first fed to an extrusion apparatus, and the liquid agent is sprayed or otherwise contacted with the polymer before the two are blended with one another through the action of the mixing elements of the apparatus.