Electrically and thermally conductive resins can be produced by adding conductive fillers to insulating polymers. Mechanical properties, such as tensile modulus, are also important. This research focused on performing compounding runs followed by injection molding and tensile testing of carbon-filled nylon 6,6 and polycarbonate-based resins. The three carbon fillers investigated included an electrically conductive carbon black, synthetic graphite particles, and a milled pitch-based carbon fiber. For each polymer, resins were produced and tested that contained varying amounts of these single-carbon fillers. In addition, combinations of fillers were investigated by conducting a full 2(3) factorial design and a complete replicate in each polymer. These tensile modulus experimental results were then compared to results predicted by several different models. For the composites containing only one filler type, the Nielsen model with the modified Psi term provided the best prediction of the actual experimental values. For the composites containing more than one filler type, a new parameter, which includes the vibrated bulk density (VBD) of the fillers, was incorporated into the Nielsen model with the modified Psi term. This model with the new VBD parameter provided the best estimate of experimental tensile modulus for composites containing multiple-filler types. (C) 2003 Wiley Periodicals, Inc.