Most premature failure of underground crosslinked polyethylene (XLPE) cables in service, a matter of great concern, is due to aging induced by water treeing. To improve the water-tree resistance, sodium-neutralized poly (ethylene-co-acrylic acid) (EAA-Na) ionomers were blended with XLPE; the EAA-Na ionomers were prepared through the neutralization of sodium hydroxide and poly(ethylene-co-acrylic acid). A series of XLPE/EAA-Na ionomer blends were investigated through the measurement of the water absorption ratio, water treeing, and mechanical and dielectric testing; the results strongly suggested that EAA-Na ionomers could improve the water-tree resistance of XLPE, and the XLPE/EAA-Na blends retained excellent mechanical properties and dielectric properties. Moreover, through the characterization of XLPE/EAA-Na blends with Fourier transform infrared spectrometry, dynamic mechanical analysis, and scanning electron microscopy, it was found that the neutralization reaction could be achieved completely; the XLPE and EAA-Na ionomers were partially compatible, so the EAA-Na ionomers could be dispersed well in the matrix with the process examined in this study. (c) 2006 Wiley Periodicals, Inc.