Partial vapour pressure of oxygen in contact with materials is considered as a key parameter which influences the oxidation processes stimulating the formation of variety of oxidation products. In this work manganese oxide nanorods; Mn5O8 and β-MnO2 have been prepared by isothermal decomposition of manganite (28 nm) precursor in air. Calcination temperature and amount of oxygen accessible to the precursor are crucial parameters that affect the decomposition pathway. X-ray single phase Mn5O8 is obtained by calcination of manganite at 350 °C in small size glass tube for 1 h; the case in which slight amount of oxygen is in close contact with the surface of the precursor. Using large size tube β-MnO2 is produced. Morphological investigations showed that the precursor has rod like morphology and the transformations to the corresponding oxide are isomorphous approaches. The studies of the magnetic properties of Mn5O8 showed the existence of tiny amount of impurities related to the presence of the spinel Mn3O4 (0.4%) into the sample. Presence of these impurities is explained on the base of accumulation of some unreacted Mn(OAc)2 over the surface of manganite precursor which is evidenced by IR analysis. By annealing of manganite at higher temperatures, Mn(OAc)2 decomposes leading to formation of the spinel.