- Global climate changes, due to rise of combustion of fossil fuels and greenhouse gas emissions, are becoming a major environmental concern, which can be approached by utilizing of waste heat into usable electrical energy, as in thermoelectric, TE, heat to electricity conversion. Among the recently developed TE materials classes, half-Heusler, HH, alloys gain a lot of attention due to their combined high electronic TE potential with a high temperature stability. So far the most efficient HH compositions are n -type. In the current research, based on ab-initio DFT and statistical thermodynamic calculations as well as an experimental validation, the temperature dependent solubility limit of Sc in HH (Ti 1- c Sc c )NiSn alloys is reported. It is shown that below the solubility limit p -type TiNiSn rich single phases are expected, while beyond the solubility limit a phase separation into ScNiSn and p -type TiNiSn rich phases is favored. This finding is of a high significance upon designing of TE devices with similar n - and p - type legs, as required for practical applications, with an addition of very interesting sub-micron phase-separation features, enabling scattering of phonons and enhancing of the TE performance.