Strong acid cation-exchange membranes were obtained by radiation-induced grafting of acrylic acid and sodium styrene sulfonate onto high-density polyethylene (HDPE). Thermal and chemical properties of the cation-exchange membranes were investigated. The effectiveness of -SO3Na containing membranes was conformed in inducing high resistance to oxidative degradation. The char residue of the grafted HDPE is greater than that of ungrafted HDPE. It shows that the branch chains, including -SO3Na and -COOH groups, give catalytic impetus to the charring. The crystallinity of the grafted membranes was decreased when increasing the grafting yield. It was assumed that the decreased crystallinity was due to collective effects of the inherent crystallinity dilution by the amorphous grafted chains and the crystal distortion of the HDPE component. (c) 2006 Wiley Periodicals, Inc.