The B2 → B19′ martensitic transformation in a quenched Ni-rich NiTi alloy was found to proceed under isothermal conditions at temperatures both within and outside of the transformation temperature range as observed during continuous cooling measurement. The variation in the volume fraction of the martensite that appeared during isothermal holding of quenched Ni-rich NiTi alloy at different temperatures T^∗ was measured. It was found that the maximum volume fraction of the martensite (45%) appeared during holding at M_s temperature for 30 min. It was found that the kinetics of the isothermal transformation was different for T^∗ > M_s and T^∗ < M_s due to the different structure of the alloy before holding that might influence the mechanisms of the martensite formation.