Abstract In a series of experiments to evaluate the possibility of reducing the lattice thermal conductivity in silicon-germanium alloys through the formation of an inert, intragranular nanophase, a number of p-type Si-20 at.% Ge alloys, nominally doped with 0.5 at.% boron, were prepared with varying amounts of fullerite, a mixture of 90% C 60 + 10% C 70. The fullerite, in the form of a fine black powder with a particle size of 0.7 nm, was shown by X-ray diffraction (XRD) to be nearly amorphous. The alloys were synthesized by mechanical alloying (MA) and the fullerite was added during various stages of preparation. Following consolidation by hot pressing, the compacts were found to be fully dense and homogeneous. Slices of each compact were characterized by the Hall effect at room temperature and also by electrical resistivity, the Seebeck coefficient, and thermal diffusivity measurements to 1000 °C. Addition of 0.2–0.8 wt.% fullerite by a simple mechanical blending operation resulted in an overall decrease in the carrier concentration of up to 35% in alloys hot pressed at 1200 °C, compared with 3.0 × 10 20 cm −3 for baseline alloys without fullerite additions. Higher hot-pressing temperatures resulted in both an increase in the carrier concentration and carrier mobility. A reduction in thermal conductivity of up to 22% compared with standard p-type alloys was observed in samples containing 0.8 wt.% additions. In this study, a maximum integrated average figure of merit, Z, between 300 and 1000 °C of 0.65 × 10 −3 °C −1 was obtained, corresponding to 0.4 wt.% addition of fullerite, which is 30–35% higher than that of ‘standard’ p-type Si-Ge alloys doped with 0.25 at.% boron prepared by conventional methods. Larger amounts of fullerite caused a decrease in Z. Observation of selected samples by transmission electron microscopy (TEM) revealed that the fullerite reacted with silicon to form nanophase SiC inclusions.