This article reports on the making and characterization of composite materials prepared by compression molding of a commercial grade thermosetting resin of urea-formaldehyde filled with a-cellulose in powder form mixed successively with carbon black, synthetic graphite, and activated carbon. The morphology of the constituents and the composites has been characterized by optical microscopy. The porosity effect has been discussed from density measurements. Furthermore, it has been shown that the hardness of the samples remains almost constant with the increase of filler concentration. The electrical conductivity shows clearly a non-linear behavior. The observed values are lower than 10(-11) S/cm, unless the filler content reaches the percolation threshold beyond which the conductivity increases markedly by as much as ten orders of magnitude, indicating insulator-conductor phase transition. The conduction threshold depends on the filler nature. The results have been interpreted by means of the statistical percolation theory. (c) 2005 Wiley Periodicals, Inc.