Thermal martensite stabilization has been studied for many years in several shape memory alloys (SMA) and some general mechanisms have been proposed to explain this phenomenon. However, Ni-Ti based alloys have been thought not to undergo this effect, although mechanically induced stabilization of the martensite has been observed in these alloys. As a thermally activated process, an increase in the ageing temperature and/or time will help to induce thermal stabilization. Ternary additions to Ni-Ti based alloys, like Hf or Zr, have managed to raise considerably the transformation temperatures, in such a way that it is possible to age these materials in martensite at reasonably high temperatures. Confirmation of the thermally induced stabilization of martensite in this family of SMA is presented in this work. Ageing treatments in martensite between 393K and 433K during times from few hours up to several months have been performed in Ni-Ti-Hf bulk and melt-spun ribbons, all resulting in martensite stabilization. After them, the typical transient shift of the first reverse transformation temperatures bas been observed. The results will be discussed in terms of the already proposed models for martensite stabilization.