Polymeric positive temperature coefficient (PTC) materials have been prepared by incorporating carbon black (CB) into two different polymer matrices, crystalline high density polyethylene (HDPE) and amorphous polystyrene (PS). The effects of thermal volume expansion on the electrical properties of conductive polymer composites were studied. The volume fraction of conductive particles behaves like a switch from insulator to conductor in the polymeric PTC composite. Our mathematical model and experimental model have proved that the abrupt resistivity increase at PTC transition range and at the percolation curve close to the critical volume fraction for both polymeric PTC composites have the same conductive mechanism. The thermal expansion is one of the key factors responsible for the PTC effect and can be seen by comparing the PTC transition curves from model predictions and experiment. Furthermore, the model predicts PTC curves of CB/PS composite more successfully than it does for the CB/HDPE composite, and the reasons for this are also discussed. (c) 2007 Wiley Periodicals, Inc.