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A theoretical model has been developed (Klemens 1987) that predicts that the addition of ultra-fine, inert, phonon-scattering centers to SiGe thermoelectric material will reduce its thermal conductivity and improve its figure-of-merit. To investigate this prediction, ultra-fine particulates (20Å to 200Å) of boron nitride have been added to boron doped, p-type, 80/20 SiGe. All previous SiGe samples produced from ultra-fine SiGe powder without additions had lower thermal conductivities than standard SiGe, but high temperature (1525K) heat treatment increased their thermal conductivity back to the value for standard SiGe. However, the SiGe samples with inert boron nitride or silicon nitride, phonon-scattering centers retained the lower thermal conductivity after multiple heat treatments at 1525K.