Acrylic acid and sodium styrene sulfonate were grafted onto high-density polyethylene (HDPE) membrane by preirradiation, and we prepared a cation exchange membrane with strong acid and weak acid groups. The HCl was used as an additive to change the pH of grafting system, and the effect of pH on grafting yield was studied. In simultaneous radiation method, the overall grafting yield (G(s)) and grafting yield of SSS onto HDPF (G(s)) increased with decreasing of pH. In preirradiation method, G(t) and G(s) first increased with decreasing of pH, and the highest grafting yield was observed at pH = 2.80, then decreased if we further reduced the pH of the grafting system. Viscosity of grafting system was changed owing to the change of the ionization degree of polyacrylic acid in the presence of HCl, so that diffuse rate of monomers into matrix was affected. (c) 2006 Wiley Periodicals, Inc.