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Much effort has been made to synthesize porous NiTi alloys using powder metallurgy techniques. However, the sintered products from elemental Ti and Ni powders often contain Ti2Ni and Ni3Ti in addition to the desired NiTi phase, and the thermal and mechanical properties of the sintered products are inferior comparing to cast or wrought NiTi alloys. This study investigated the solid-state diffusion reactions between elemental Ti and Ni powders during sintering to delineate the origin of the formation of these undesired intermetallic phases and proposed a physical model to explain the phase formation processes. The intermediate diffusion reaction products include Ti(Ni) and Ni(Ti) solid solutions, Ti2Ni, Ni3Ti and NiTi. The Ti2Ni and Ni3Ti intermetallic phases are the primary reaction products between Ti and Ni, and NiTi phase is formed only as a secondary reaction product from Ti2Ni and Ni3Ti. In addition, two NiTi phases of different Ni contents are formed from either Ti2Ni or Ni3Ti. These findings clarify some uncertainties and common misunderstanding on NiTi formation through elemental powder sintering and provide a guide for the design of powder metallurgy of Ti and Ni.