The thermal resistivity, Seebeck coefficient, electrical resistivity, and Hall mobility of Ge‐Si alloys have been measured throughout the Ge‐Si alloy system as functions of impurity concentration in the range 2×1018−4×1020cm−3, and of temperature in the range 300°–1300°K. A qualitative interpretation of these properties is given. For power conversion, boron and phosphorus were found to be usefulp‐type andn‐type impurities, respectively, because of their high solid solubilities. At 1200°K, the maximum values of the dimensionless figure of meritzTwere 0.8 forp‐type Ge0.15‐Si0.85alloy doped to 2.1×1020cm−3holes, and 1.0 forn‐type Ge0.15‐Si0.85alloy doped to 2.7×1020cm−3electrons. The maximum over‐all efficiency of a stable generator operating between 300°–1200°K, using the bestp‐type andn‐type materials was computed to be 10%.