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Structure, transport and magnetic properties have been investigated for the layered materials Na γ Co 1−x Mn x O 2 (0 x 0.5). With increase of the Mn concentration, the in-plane lattice parameter a decreases while the lattice parameter c increases. 3% Mn doping can result in a metal-insulator transition. The temperature dependence of the magnetization of the samples obeys the Curie-Weiss law and the effective moment of the magnetic ions increases with doping. Electron energy-loss spectroscopy measurement shows that the Co valence is about 3.3 and remains unchanged upon doping. However, the valence of Mn increases rapidly with doping and reaches 3.7 ± 0.1 for the sample with x = 0.5. The results are discussed in terms of the disorder effect induced by doping of magnetic ions. This work demonstrates that the structure, transport and magnetic properties of Na γ CoO 2 are very sensitive to doping with magnetic ions.