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In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ′ precipitate morphology, the γ/γ′ lattice misfit and the γ/γ′ microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900° C for 50 h, reduces the γ/γ′ lattice misfit by∼ 40%(from+ 0.32% to+ 0.19%, measured at room temperature) and hence alters the γ′ morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ′ interface by atom probe tomography (APT) reveal Re partitions to the γ phase (K R e= 0.34) and also results in the partitioning reversal of Mo to the γ phase (K M o= 0.90) from the γ′ precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (Γ R e, ranging from 0.8 …