The pseudoternary (Ge,Pb,Sn)Te system is characterized by demixing to both Pb- and Ge- rich telluride submicrometer and nanosized domains. The present study is concerned with the thermoelectric (TE) properties of the quasi-ternary (Ge0.6Pb0.3Sn0.1)Te compound at 390 °C, during the demixing process over lengthy periods of time up to 695 h. The dimensionality of the various physical metallurgy patterns evolved was correlated to the lattice thermal conductivity, κL, values. Excluding an initial period (<90 h) of increasing κL, associated with a coarse lamellar structure formation, an opposite decreasing trend (down to ∼0.8W/mK after 695 h), associated with nucleation of submicrometer (∼200 nm) circular phases, twinning and dislocation networks, all considered as effective phonon scattering centers, was observed. Expansion of these submicrometer sites, all over the samples, during the heat treatments, without any subsequent coarsening, clearly demonstrates the potential of retaining, or even improvement of th...