The thermoelectric properties between 10 and 300 K and the growth of single crystals of n-type and p-type GeBi₄Te₇, GeSb₄Te₇ and Ge(Bi_1−xSb_x)₄Te₇ solid solution are reported. Single crystals were grown by the modified Bridgman method, and p-type behavior was achieved by the substitution of Bi by Sb in GeBi₄Te₇. The thermopower in the Ge(Bi_1−xSb_x)₄Te₇ solid solution ranges from −117 to +160 µV K⁻¹. The crossover from n-type to p-type is continuous with increasing Sb content and is observed at x ≈ 0.15. The highest thermoelectric efficiencies among the tested n-type and p-type samples are Z_nT = 0.11 and Z_pT = 0.20, respectively. For an optimal n-p couple in this alloy system the composite figure of merit is Z_npT = 0.17 at room temperature.