The conductivity of an immiscible polymer blend system, microfibrillar conductive poly(ethylene terephthalate) (PET)/polyethylene (PE) composite (MCPC) containing carbon black (CB), was changed by the addition of insulating CaCO3 nanoparticles. In MCPC, the PET forms microfibrils during processing and PE forms the matrix. The CB particles are selectively localized in the PET microfibrils. When the insulating CaCO3 nanoparticles are added, they substitute for some of the conductive CB particles and obstruct the electron paths. As a result, the resistivity of the MCPC can be tailored depending on the insulating filler content. The resistivity-insulating filler content curve displays a sluggish postpercolation region (the region immediately following the percolation region and in front of the equilibrium flat of the resistivity-filler content curve), suggesting that the MCPC in the postpercolation region possesses an enhanced manufacturing reproducibility and a widened processing window. These features are of crucial importance in making sensor materials. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3073-3079, 2008