The thermoelectric properties between 10 and 300 K and the growth of single crystals of n-type and p-type GeBi(4)Te(7), GeSb(4)Te(7) and Ge(Bi(1-x)Sb(x))(4)Te(7) 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 GeBi(4)Te(7). The thermopower in the Ge(Bi(1-x)Sb(x))(4)Te(7) 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(n)T = 0.11 and Z(p)T = 0.20, respectively. For an optimal n-p couple in this alloy system the composite figure of merit is Z(np)T = 0.17 at room temperature.