In this work, we investigated the effect of carbon black (CB)-loading on the miscibility of poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends using the CB particle as a self-diagnosing probe. The mobility of the CB particles both in homogeneous and PVDF/PMMA blend matrices was real-time monitored through in situ recording the variation of electrical resistivity with time during annealing treatments. It was found that the dynamic percolation curves in a homogeneous matrix behave with self-similarity to different annealing temperatures and CB concentrations, and the activation energy of the percolation rime is near to that of the zero-shear-rate viscosity of polymer matrix regardless of the filler concentrations. However, in the CB-filled PVDF/PMMA system, both the shape of the dynamic percolation curves and the activation energy of the CB mobility were found to change with the annealing temperature. These facts implicate that the PVDF/PMMA blend matrices filled with CB are not homogeneous. SEM and AFM observations as well as DSC and DMA studies confirmed that the incorporation of CB induces the phase fluctuations in PVDF/PMMA miscible blends due to a preferential adsorption of PVDF molecules on the CB surface. As a result, the CB particles are: heterogeneously distributed in the miscible blends and thus greatly reduce the percolation threshold of the PVDF/PMMA/CB composites. (C) 2001 Elsevier Science Ltd. All rights reserved.