and it involves the simultaneous manipulation of the TE power (absolute Seebeck coefficient) S, the electrical conductivity r, and the thermal conductivity j. The search for efficient TE materials mainly focuses on degenerate semiconductors since the underlying physics of these systems allow the coexistence of high thermopower values with high electrical conductivity to achieve high power factors: PF = S r. The Seebeck coefficient is inversely related to the electrical conductivity according to the Boltzmann transport equation, and, as a result, maximization of one cannot be achieved without minimization of the other. An interesting alternative that has been recently suggested to achieve high power factors is the quantum-confinement effect; however, definite experimental verification of this is still lacking. Another route to achieving high-performance TEs is through the minimization of the thermal conductivity. To this end, many suggestions have been made to increase ZT. These include the phonon-glass electron-crystal approach (where loosely bound atoms rattle in cage structures) as in clathrates, and the thin-film multilayer approach where the introduction of interfaces significantly reduces phonon propagation. Indeed, artificial thin-film superlattice structures grown by molecular-beam epitaxy (MBE) like Bi2Te3/ Sb2Te3 [6] and PbSe0.98Te0.02/PbTe [7–9] exhibit very low thermal conductivities and, as a result, enhanced ZT values. The key feature of these systems is the large number of interfaces introduced by the inherent nanofabrication technique that in turn reduce the phononic part of j through interface phonon scattering. Interestingly, “nanocomposites” in bulk form have been recently identified in the n-type AgPbmSbTe2 + m systems where compositional fluctuations at the nanoscopic level, resulting in a distinct type of nanostructuring, seem to play a key role in the previously reported very low thermal conductivity. In contrast to the thin-film multilayers, bulk nanocomposite systems offer the advantages of large-scale industrial production and the sustenance of large thermal gradients for extended time. The challenge, therefore, lies in identifying equally efficient p-type materials so that they can be employed in the fabrication of TE modules. Here we report on the Ag(Pb1 – ySny)mSbTe2 + m series and show that certain compositions exhibit high performance p-type TE properties (e.g., ZT ∼ 1.45 at 630 K) as a result of their very low thermal conductivity. We show as well that the Ag(Pb1 – ySny)mSbTe2 + m systems are in fact bulk nanocomposites. We demonstrate that varying the m and y values, as well as the Ag and Sb concentrations, allows for control over a wide range of properties such as carrier concentration, TE power, and thermal conductivity. These exceptional properties, derived from specific compositions, outperform the standard state-of-the-art p-type systems like TAGS ((AgSbTe2)0.15(GeTe)0.85, ZT ∼ 1.2 at 720 K), PbTe (ZT ∼ 0.7 at 740 K), and Zn4Sb3 (ZT ∼ 1.3 at 670 K). The electronic-transport properties of the Ag(Pb1 – ySny)mSbTe2 + m system can be tuned primarily through carefully controlling the Pb/Sn ratio, i.e., the y value, and secondarily the Ag and Sb composition. Increasing the Sn concentration results in enhanced electrical conductivity and a subsequent low TE voltage response. On the other hand, we have found that the TE power varies approximately as a linear function of y, and, at 650 K, can be tuned to reach as high as ∼ 280 lV K. We find that the functional dependence of the electrical conductivity on temperature does not change dramatically with y or m and scales as T, where 1.8 ≤ n ≤ 2.25 from room temperature up to 640 K. Therefore, the electrical conductivity at 600 K is roughly 22–28 % of the room-temperature electrical conductivity for all y. It is important to mention that there was C O M M U N IC A TI O N S