The purity and structural stability of the high thermoelectric performance Cu 12 Sb 4 S 13 and Cu 10.4 Ni 1.6 Sb 4 S 13 tetrahedrite phases, synthesized by solid–liquid–vapor reaction and Spark Plasma Sintering, were studied at high temperature by Rietveld refinement using high resolution X-ray powder diffraction data, DSC/TG measurements and high resolution transmission electron microscopy. In a complementary study, the crystal structure of Cu 10.5 Ni 1.5 Sb 4 S 13 as a function of temperature was investigated by powder neutron diffraction. The temperature dependence of the structural stability of ternary Cu 12 Sb 4 S 13 is markedly different to that of the nickel-substituted phases, providing clear evidence for the significant and beneficial role of nickel substitution on both sample purity and stability of the tetrahedrite phase. Moreover, kinetic effects on the phase stability/decomposition have been identified and discussed in order to determine the maximum operating temperature for thermoelectric applications. The thermoelectric properties of these compounds have been determined for high density samples (>98%) prepared by Spark Plasma Sintering and therefore can be used as reference values for tetrahedrite samples. The maximum ZT of 0.8 was found for Cu 10.4 Ni 1.6 Sb 4 S 13 at 700 K.