The electrochemical extraction and insertion of Na in P2-Na-2/3[Ni1/3Mn2/3]O-2 was studied by in situ X-ray diffraction. All the original Na can be extracted from P2-Na-2/3[Ni1/3Mn2/3]O-2 and it can be reversibly inserted again. When x > 1/3 in Na-x[Ni1/3Mn2/3]O-2, the compound remains in the P2 structure. For x approximate to 1/3, a small amount of O-2-type stacking faults are introduced into the structure. When x < 1/3, the electrode exists as two coexisting phases. These are P2-Na-1/3[Ni1/3Mn2/3]O-2 (with some O-2-type stacking faults) and [Ni1/3Mn2/ 3]O-2. [Ni1/3Mn2/3]O-2 adopts the O-2 structure with stacking faults. As sodium is reinserted in [Ni1/3Mn2/3]O-2 to make Na-1/3[Ni1/3Mn2/3]O-2 the P2 structure forms again. The evolution of the lattice parameters of P2-Na-x[Ni1/3Mn2/3]O-2 (1/3 < x < 2/3) with x and the lattice constants of [Ni1/3Mn2/3]O-2 are also reported.