In this study, three different carbon fillers (Thermocarb TC-300 synthetic graphite, Ketjenblack EC-600 JD carbon black, and Hyperion Catalysis International's FIBRIL (TM) carbon nanotubes) were added to a polypropylene matrix to produce single filler composites with filler concentrations of up to 80 wt % synthetic graphite (61.6 vol %), 15 wt % carbon black (8.1 vol %), and 15 wt % carbon nanotubes (7.4 vol %). The through-plane thermal conductivity for each formulation was measured. For the synthetic graphite, carbon black, and carbon nanotubes composites, the Nielsen model was applied to the experimental through-plane thermal conductivity data. The Nielsen Model presented in this work showed very good agreement with experimental data. The model parameters were similar to those used in the literature for these fillers in other polymers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 3261-3267, 2009