- Thermoelectrics are known as one of the emerging renewable power generation technologies. Half-Heusler based semiconducting intermetallic compounds show high potential as thermoelectric materials due to the abundance of their elements in nature, their high mechanical and chemical stability, and their favorable electronic properties. Their main limitation lies in their high lattice thermal conductivity, κl. In the current research, the potential of κl reduction due to generation of composites based on phase separation of off-stoichiometric Ti0.3Zr0.35Hf0.35Ni1+δSn alloys into half- and full-Heusler composites was investigated. Due to the strong metallic nature of the full-Heusler phase, its electronic effect on the thermoelectric transport properties under various morphological and compositional conditions was analyzed by the general effective media (GEM) approach. It was shown that although a major κl reduction of up to ∼37% was associated with phonon scattering by the embedded full Heusler phase, the electronic properties for the parallel-like alignment of this phase, following arc melting and spark plasma sintering, deteriorated with increase of the relative amount of this phase. Therefore a maximal ZT enhancement of ∼41%, compared to the uni-phase HH stoichiometric composition, was obtained for the minimal (δ = 0.01) deviation of the stoichiometry examined, corresponding to the minimal relative amount of the scattered FH phase inside the HH matrix.