Electron transmission microscopy was used to study B2 → Ni5Al3 ordering in the β solid solution of β/γ Ni-Co-Cr-Al eutectics in the process of prolonged annealings at 550°C. The formation of the Ni5Al3 superstructure of a lower symmetry than that of the initial B2 superstructure is shown to occur by the mechanism of heterogeneous nucleation and growth in the β matrix of disk-shaped Ni5Al3 particles possessing microdomain internal structure. An increase in the chromium concentration in the alloy from 14 to 20 wt % noticeably retards the growth of Ni5Al3 particles. Cobalt in concentrations from 0 to 15 wt % exerts no significant effect on the kinetics of the B2 → Ni5Al3 transformation, but reduces the intensity of the concurrent decomposition of the supersaturated β solid solution with the precipitation of α-Cr particles.