The thermoelectric properties of 28 sintered Si0.8 Ge0.2 alloys, heavily doped with either boron or phosphorus and prepared from powders with median particle sizes ranging from about 1 μm to over 100 μm, have been determined from 300 to 1300 K. The thermal conductivity decreases with decreasing particle size, however, the figure of merit is not significantly increased due to a compensating reduction in the electrical conductivity. The thermoelectric figure of merit is in good agreement with results of Dismukes et al. [J. Appl. Phys. 10, 2899 (1964)] on similarly doped alloys prepared by zone-leveling techniques. The electrical and thermal conductivity are found to be sensitive to preparation procedure while the Seebeck coefficient and figure of merit are much less sensitive. The high-temperature electrical properties are consistent with charge carrier scattering by acoustic or optical phonons.