Microstructural features of the heat-resistant β phase in γ/β eutectics of the Co-Al and Co-Cr-Al alloys were studied. It was shown that the eutectic β phase in the binary alloy is thermally unstable. Microsegregation occurs in this phase upon high-temperature annealing (1100°C) and, upon subsequent cooling, the phase decomposes with the formation of fine γ-phase particles in which thin interlayers of the ε phase form due to bcc → hep polymorphic transformation. Alloying with 10-20 wt % of chromium suppresses this decomposition and substantially increases the stability of the β phase in the Co-Cr-Al γ/β eutectics.