Structural and phase transformations in the main phase constituents of the β/γ eutectic Ni-Co-17 wt % Cr-Al alloys containing up to 30 wt % Co were studied by transmission electron microscopy. Main factors causing the destabilization of the β and γ phases upon cooling of the eutectics from high temperatures or during their annealing at 450 and 600°C were established. The thermal stability of the β and γ phases was shown to increase with increasing cobalt content up to 30 wt %.