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We have investigated the doping effects of elements (Sn, Pb, Ge) with valence 4 in Bi-Sb alloys. The solubilities of these elements in Bi-Sb alloy are known to range from ∼3 to ∼0.5 to zero atomic percent, respectively. We employed a rapid solidification method to prepare samples that allowed us to achieve a high doping level of Ge and a high doping efficiency of Pb in Bi85Sb15. Using this method, unprecedented high Ge content (up to 13 atomic percent) could be solutionized in Bi-Sb. Microstructure analyses and thermoelectric transport property measurements have been performed on the doped samples which all showed p-type properties at cryogenic temperatures. The doping efficiency of the elements (Sn, Pb, Ge) was calculated based on the Hall measurements and discussed in light of donor and acceptor sites in Bi-Sb. A two-band effective mass model was used to simulate the thermoelectric transport properties within the Boltzmann transport theory. The present study revealed a gap-decreasing phenomenon in the undoped and doped samples. This gap-decreasing behavior is a heretofore unrealized obstacle to achieving a high thermoelectric figure of merit in p-type Bi-Sb alloys.