Phase formation of NiTi alloys via solid-state diffusion reactions is investigated. Samples of different Ni-Ti compositions were synthesized from elemental powders. The study revealed that the sintered samples consisted of Ti(Ni), NiTi2, NiTi and Ni3Ti in co-existence. Such co-existence is not expected from the equilibrium phase diagram, but can be explained in terms of Ni/Ti inter-diffusion processes. Thermodynamic analysis indicates that formation of NiTi is not favoured in primary reactions between Ni and Ti, but can be formed via secondary reactions involving primary reaction products of NiTi2 and Ni3Ti. Such reactions are difficult in solid state due to the difficulties of long-distance diffusion required. The synthesized alloys were found to exhibit much reduced martensitic transformation intensity, implying low transformation volume.