The present article is concerned with the temperature-conductivity characteristics of blends consisting of polyisothianaphthene (PITN) particles and a soluble poly(3-hexylthiophene) (P3HT). PITN was synthesized by direct conversion of 1,3-dihydroisothianaphthene (DHITN) monomer using N-chlorosuccinimide (NCS) as an oxidation/dehydrogenation reagent. The high conductivity and thermal stability of the doped and dedoped PITN were confirmed. Microscopic investigation by scanning electron microscopy (SEM) showed that the as-prepared PITN exhibited diversified shapes and sizes, with large rectangular particles having an average size of 2 similar to 5 mu m and fine round particles ranging from 0.1 to 0.3 mu m. The PITN particles were blended with the chemically synthesized P3HT as a high conductivity component to improve the conductivity and simultaneously maintain the positive temperature coefficient (PTC) effect of the original P3HT near its melting point. The temperature-conductivity characteristics for PITN-P3HT blends with various PITN contents showed that a blend having both a high conductivity (nearly 3 similar to 4 orders higher than that of the original P3HT) and a good PTC intensity could be obtained with a PITN content of 20 similar to 25%. The different temperature-conductivity behavior of P3HT blends filled with PITN as compared to other conducting particles, for example, carbon black, was explained by its unique dispersion structure due to a relatively higher adhesive interaction of PITN particles with the P3HT matrix during the precipitation process. The results from heating recycles revealed that the PTC effect of PITN-P3HT blends was not just related to the conductivity decrease of the P3HT matrix, arising from the conformational change of the conjugated backbone during the melting, but also to the dilution effect of the conducting percolation network due to the mobility of PITN particles induced by the viscosity decrease of the P3HT matrix. (C) 2005 Wiley Periodicals, Inc.