The preparation of nanocrystalline (n-) Mn-oxides has been investigated. The oxides were obtained by oxidation of n-Mn metal, prepared by inert gas condensation, in air and in oxygen at different temperatures and times, Differential thermal analysis revealed a broad exothermic peak with a maximum at 596 degrees C, while thermogravimetry showed the adsorption of O-2 between 580 and 700 degrees C. XRD diffractograms, performed after annealing at lower temperatures in air (T < 400 degrees C) for 1 h, revealed the formation of MnO and Mn3O4 as main crystalline phases. After oxidizing between 400 and 600 degrees C the presence of Mn5O8 and Mn2O3 has been detected, while at higher temperatures (T greater than or equal to 600 degrees C) Mn2O3 as single phase was observed. After oxidation the average particles size increased from 27 to 43 nm but no further changes of the crystalline phases was observed.