Carbon nanofibers of diameter 200 nm may be used as an additive to thermoplastics for applications requiring electrical conductivity and enhanced mechanical properties. The electrical properties of nanofiber in thermoplastics such as nylon and polypropylene and are very attractive compared with those provided by other conventional conducting additives. Because of the low diameter of the nanofibers used, the onset of electrical conductivity (percolation threshold) can be below 1 volume %. Because of the highly conductive nature of the fibers, particularly after a graphitization step, the composites can reach resistivities as low as 0.15 Ohm cm. These conducting composites may be used for applications such as radio frequency interference shielding, primerless electrostatic painting, and static discharge. In order to make composites having excellent mechanical properties, good adhesion between fiber and matrix is essential. Carbon nanofiber-matrix adhesion was studied after surface treating the fibers using a variety of methods. Among as-grown fibers, those produced with longer gas phase feedstock residence times in the fiber growth reactor were less graphitic but adhered to a polypropylene matrix better, giving improved tensile strength and modulus Two chemical treatments were found to be somewhat effective in increasing tensile strength, but both decreased the modulus.. A modest degree of oxidation was also found to increase adhesion to the matrix and increase composite tensile strength, while extended oxidation attacked the fibers sufficiently to decrease composite properties.