A comprehensive study of GeTe-based materials where Ge is replaced by 2 at.% Ag, 2 at.% Sb, and additionally by 1 or 2 at.% Dy (Ag2Sb2Ge46 − xDyxTe50 alloys) has been conducted using Seebeck coefficient, electrical resistivity, and thermal conductivity measurements in the temperature range of 300–750 K, and X-ray diffraction (XRD) and 125Te NMR at 300 K. According to our data, at ∼720 K, GeTe, a self-doping degenerate semiconductor, exhibits the highest known power factor (PF) among tellurides, 42 μW cm−1 K−2, but due to high thermal conductivity, the thermoelectric figure of merit, ZT, is relatively low, ∼0.8. Replacement of Ge in GeTe by [Ag + Sb] and Dy enhances the Seebeck coefficient and produces a small increase in carrier concentration deduced from 125Te NMR spin–lattice relaxation. The PF of Ag2Sb2Ge45Dy1Te50 at ∼720 K is found to be still high, 40 μW cm−1 K−2, and due to a reduction in thermal conductivity, ZT reaches 1.2. Our data show that relatively high ZT values in tellurides can be obtained primarily due to high PF while thermal conductivity is moderate. The enhancement of the Seebeck coefficient observed in Ag2Sb2Ge46 − xDyxTe50 alloys can be attributed to energy filtering due potential barriers formed by [Ag + Sb] and rare-earth Dy atoms.