Conductive polymer composites used as candidates for positive temperature coefficient (PTC) materials are faced with performance decay characterized by gradually increased room-temperature resistivity and decreased PTC intensity. Considering that deterioration of the properties is mainly related to the capability of conductive networks established by conductive fillers to recover from the effect of repeated expansion/contraction in a timely manner, the present work introduces chemical bonding into the filler/matrix interphase. The experimental results indicate that in the composites consisting of conductive carbon black (CB), low-density polyethylene (LDPE), and ethylene-vinyl acetate copolymer, CB particles can be covalently connected with LDPE through melt grafting of acrylic acid. As a result, the composites are provided with reduced room-temperature resistivity and significantly increased PTC intensity. Compared with the composites filled with untreated CB, the present composites possess reproducible PTC behavior and demonstrate stable electrothermal output in association with negligible contact resistance at the composites/metallic electrodes contacts. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2438 -2445, 2003.