Heterogeneous materials composed of conducting YBa2Cu3O7-x particles randomly dispersed inside an insulating low density polyethylene (LDPE) matrix were prepared. The resistivity of the composites was measured as a function of temperature between 293 and 400 K. A large positive temperature coefficient (PTC) of resistance was observed near 395 K. The mechanism of the PTC effect in the materials is related to the percolation behavior of the YBa2Cu3O7-x/LDPE composites and the thermal expansion of the LDPE matrix. The intensity of the PTC effect for these composites was found to be as large as nine orders of magnitude. POLYM. ENG. SCI., 49:1666-1669, 2009. (C) 2009 Society of Plastics Engineers