A conductive polymer composite was prepared by melt-mixing of an immiscible semicrystalline polymer blend of an alternating copolymer of tetrafluoroethylene-ethylene (ETFE), high density polyethylene (HDPE), and carbon black (CB). The optical microscopy and time-of-flight secondary mass spectrometry results indicated that the CB particles were selectively localized in the HDPE phase. In addition, it was found that the CB-filled HDPE particles formed a dispersed phase in the ETFE matrix. A double-positive temperature coefficient (PTC) effect was observed in the composite, caused by the large thermal expansion due to the consecutive melting of HDPE and ETFE crystallites. The negative temperature coefficient (NTC) that was observed in this system could not have been caused by the formation of flocculated structures because the size of the CB-filled HDPE particles is significantly large, so that their mobility is extremely limited even at high temperatures. This conclusion was confirmed by observing the morphology of the composite at temperature ranging from 25 to 250 degrees C. These results suggest that new mechanisms need to be uncovered to explain the NTC effect of conductive polymer composites. (C) 2000 Elsevier Science Ltd. All rights reserved.