Temperature and time dependence of conductive network formation in vapor-grown carbon fiber (VGCF) filled high-density polyethylene (HDPE)/poly(methyl methacrylate) (PMMA), VGCF and ketjenblack (KB) filled HDPE/isotactic polypropylene (iPP) blends have been investigated. It is found that the filled conductive polymer composites are thermodynamically non-equilibrium systems, in which the conductive network formation is temperature and time dependent, a concept named as dynamic percolation is proposed. When the composites are annealed at a temperature above the melt point of polymer matrix, the dynamic process of conductive network formation can be monitored in a real time way. Such an in situ characterization method provides more interesting information about the dispersion of conductive particles in the polymer matrix. Furthermore, a thermodynamic percolation model is modified to predict the percolation time for VGCF and KB filled HDPE/iPP multi-phase systems during the annealing treatment, and it expresses experimental results well. (c) 2006 Elsevier Ltd. All rights reserved.