The precipitation of the γ′-phase in Co-Ta-V and Co-Nb-V ternary alloys is investigated based on recent computational predictions of stable γ-γ′ microstructures. Cuboidal γ′ precipitates within a fcc-γ matrix are discovered in the Co-Ta-V system after aging for 2 h at 900 °C, and in the Co-Nb-V system after cooling from homogenization at 1250 °C. The compositions of these two new γ′-phases are measured via atom probe tomography as Co3(Ta0.76, V0.24) and Co3(Nb0.65, V0.35), respectively. Upon aging at 900 °C, the γ′ precipitates coarsen, dissolve and transform to lamellar C36-Co3(Ta,V) and needle-shape D019-Co3(Nb,V), indicating that the γ′ phase is metastable. Various alloying additions are investigated to stabilize the γ′-phase, and quinary alloys are developed, which exhibit cuboidal γ′ precipitates within a fcc-γ matrix after aging at 850 °C for 168 hours, with no precipitation of additional phases.