The mixed metal oxides having layered crystal structure have been of much interest because of their various new and excellent functions. Precise phase diagrams on the oxygen content have not been clarified for most of these layered oxides, while oxygen nonstoichiometry often plays a crucial role for determining functions of oxide materials. In this paper, we report oxygen nonstoichiometry and related physical properties mainly on the manganese-based and cobalt-based layered oxides. In the La2–2x
(x = 0.3, 0.4), dramatic change in magnetization properties with oxygen content were observed in spite of its small oxygen nonstoichiometry. For (Ca2CoO3-δ
)0.62CoO2 systems, high valence metal substitution was found to be effective for suppression of oxygen nonstoichiometry, increase of decomposition temperature and thermoelectric properties.