an electrically insulating cross - linked polyethylene composition for use in high voltage electrical cables , said cross - linked polyethylene being obtained by cross - linking a composition consisting essentially of 98 % w / w of a low density , peroxide cross - linkable polyethylene , 1 - 2 % w / w of a terpolymer of ethylene , vinyl acetate and vinyl alcohol and at least 0 . 15 % w / w of a sterically hindered amine stabiliser of said composition ; said terpolymer being obtained from about 40 - 45 % hydrolysis of an ethylene - vinyl acetate copolymer having a 20 - 30 % w / w vinyl acetate content ; and wherein said polyethylene and said terpolymer have substantially the same melt index . the cross - linked polyethylene compositions have improved resistance to moisture induced degradation .