P2-type NaMO₂ sodiated layered oxides with mixed transition metals are receiving considerable attention for use as cathodes in sodium-ion batteries. A study on solid solution (1 – y)P2-Na_xCoO₂–(y)P2-Na_xMn_2/3Ni_1/3O₂ (y = 0, 1/3, 1/2, 2/3, 1) reveals that changing the composition of the transition metals affects the resulting structure and the stability of pure P2 phases at various temperatures of calcination. For 0 ≤ y ≤ 1.0, the P2-Na_xCo_(1–y)Mn_2y/3Ni_y/3O₂ solid-solution compounds deliver good electrochemical performance when cycled between 2.0 and 4.2 V versus Na⁺/Na with improved capacity stability in long-term cycling, especially for electrode materials with lower Co content (y = 1/2 and 2/3), despite lower discharge capacities being observed. The (1/2)P2-Na_xCoO₂–(1/2)P2-Na_xMn_2/3Ni_1/3O₂ composition delivers a discharge capacity of 101.04 mAh g^–1 with a capacity loss of only 3% after 100 cycles and a Coulombic efficiency exceeding 99.2%. Cycling this material to a higher cutoff voltage of 4.5 V versus Na⁺/Na increases the specific discharge capacity to ≈140 mAh g^–1 due to the appearance of a well-defined high-voltage plateau, but after only 20 cycles, capacity retention declines to 88% and Coulombic efficiency drops to around 97%. In situ X-ray absorption near-edge structure measurements conducted on composition Na_xCo_1/2Mn_1/3Ni_1/6O₂ (y = 1/2) in the two potential windows studied help elucidate the operating potential of each transition metal redox couple. It also reveals that at the high-voltage plateau, all of the transition metals are stable, raising the suspicion of possible contribution of oxygen ions in the high-voltage plateau.