The formation of the Ni 5 Al 3 phase in multiply twinned Ni 6 5 Al 3 5 martensite was investigated by in situ transmission electron microscopy heating and high resolution electron microscopy of ex situ annealed samples. It is found that reordering in the thin foils starts around 300 [deg ]C, the domain size increasing with temperature. At each temperature the final stage is reached within a few minutes and without any change in the morphology of the martensite plates. As a result of the slight increase of about 1% in the c/a ratio, stacking faults are introduced in completely transformed samples in the small L1 0 twin variants.