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The present paper reports a new class of Co based superalloys that has γ–γ′ microstructure and exhibits much lower density compared to other commercially available Co superalloys including Co–Al–W based alloys. The basic composition is Co–10Al–5Mo (at%) with addition of 2 at% Ta for stabilization of γ′ phase. The γ–γ′ microstructure evolves through solutionising and aging treatment. Using first principles calculations, we observe that Ta plays a crucial role in stabilizing γ′ phase. By addition of Ta in the basic stoichiometric composition Co₃(Al, Mo), the enthalpy of formation (ΔH^f) of L1₂ structure (γ′ phase) becomes more negative in comparison to DO₁₉ structure. The ΔH^f of the L1₂ structure becomes further more negative by the occupancy of Ni and Ti atoms in the lattice suggesting an increase in the stability of the γ′ precipitates. Among large number of alloys studied experimentally, the paper …