The aim of this paper was to study the isothermal forward martensitic transformation in binary, ternary, and quaternary NiTi-based shape memory alloys to find the origin of this phenomenon. The results obtained showed that the isothermal B2 → B19′ transformation was observed only in the NiTi-based alloys with substitutional defects. The critical concentration of copper was found that completely suppressed the martensitic transformation both on cooling or isothermal holding in Ti-Hf-Ni-Cu alloys. The thermodynamics of the martensitic transformation that occurred in the presence or absence of the substitutional defects was discussed, and the reason for the influence of the substitutional defects on the martensitic transformation was clarified. It was verified that the formation of the martensite phase on isothermal holding was caused by the local fluctuation of the substitutional defect concentration which led to fulfilment of the thermodynamic condition for the martensitic transformation. It was shown that the maximum volume fraction of the isothermal martensite was limited by the maximum volume of martensite that corresponded to the thermal elastic equilibrium and appeared on cooling of the alloy without substitutional defects. The results allowed to conclude that the martensitic transformation remained to be thermal elastic in the NiTi-based alloys with a non-stoichiometric composition however, the martensite nucleation was controlled by the thermally activated migration of substitutional defects.