Composition, crystal structure, and stability of the thermoelectric material, known in the literature as “Zn4Sb3”, has been characterized using low- and room-temperature single-crystal X-ray diffraction techniques, as well as in situ room- and high-temperature powder X-ray diffraction methods. We have found that the Zn4Sb3 phase does not exist below 767 K (the β−γ transition temperature); it is the Zn6-δSb5 phase that is erroneously assigned the Zn4Sb3 composition and is considered to be a promising thermoelectric material. The structure of Zn6-δSb5 is similar to that of “Zn4Sb3” but no Zn/Sb mixture is observed on any Sb site. Instead, a significant deficiency on the Zn site is discovered. There are two, not one, as previously reported, Zn6-δSb5 polymorphs below room temperature. In dynamic vacuum and at elevated temperatures the Zn6-δSb5 phase becomes zinc poorer due to zinc sublimation and eventually decomposes into ZnSb and Zn before reaching its melting temperature of 841 K. The binary Zn1-δSb compound also loses zinc in dynamic vacuum and at high temperatures and decomposes into Sb and Zn. The structure of Zn1-δSb (CdSb-type) is analyzed using powder X-ray diffraction techniques.