The process of tempering of the 63.8Ni–1Co–Al martensite has been investigated by electron microscopy. A new ordered phase with the Pt₅Ga₃ type structure is found when the martensite is tempered at relatively low temperatures, 573–673 K, for several ten minutes. This tempered phase is the same in microstructure and configuration of internal defects as the as-quenched martensite except “ordering” in atomic arrangement.