Phase equilibria in the Ta‐Co‐O and Nb‐Co‐O systems have been studied at 1200°C at oxygen partial pressures from 10−0.68to 10−13.50atm for the former and from 10−0.68to 10−13.30atm for the latter. In both systems, M2CoO6and M2Co4O9are stable ternary compounds under the experimental conditions, and a new phase, Nb5Co2O14, has been identified. The Ta‐Co‐O system is simple, whereas the Nb‐Co‐O system is somewhat more complicated because of the extra phase. The lattice constants of the ternary compounds have been determined and compared with previous values. The standard Gibbs energies of reactions have been determined using oxygen partial pressures in equilibrium