Novel nanosized crystals of aquocyanophthalocyaninatocobalt (III) (Phthalcon 11) were used as a conductive filler in crosslinked epoxy materials. The crosslinked composite materials had a very low percolation threshold (phi(c), 0.9 vol %). The relationship between the volume conductivity and the filler fraction follows the scaling law of the percolation theory and suggests that the conducting particle networks were formed by random percolation of primary aggregates. The occurrence of the low pc can be explained by the presence of a fractal Phthalcon 11 particle network formed from fractal aggregates during crosslinking. The position of the percolation threshold and the volume conductivity of these crosslinked materials were found to depend heavily on the processing conditions applied. These dependencies are explained in terms of specific particle-matrix interactions and the particle-particle interactions and by taking into account different mechanisms of particle network formation. (c) 2005 Wiley Periodicals, Inc.