Compounds of Mg2Si1-x Sn (x) are environmentally friendly, inexpensive and high-efficiency thermoelectric materials for energy conversion in the temperature range 300-550 A degrees C. In this study, the thermal stability is investigated of fine powders and sintered pellets of the compounds Mg2Si0.4Sn0.6 and Mg2Si0.6Sn0.4 by heating the samples from room temperature to similar to 400 A degrees C in air, while measuring powder X-ray diffraction patterns. The diffractograms of the pellets show no significant changes upon heating for several hours, while the powder samples show increasing emergence of a Mg2Sn-rich, Mg2Si1-x Sn (x) phase, and other impurities upon heating for only several minutes. This is attributed to the larger amount of surface area in the powder samples. The appearance of the Mg2Sn-rich phase is most pronounced for the Sn-rich composition. In addition, the thermal expansion coefficients were extracted from the powder diffraction patterns. All materials have been synthesized by induction-melting followed by ball milling and spark plasma sintering. The thermal conductivity, Seebeck coefficient, electrical resistivity and Hall carrier concentrations have been measured from room temperature to 400 A degrees C on the pellets.