Nano-crystalline oxide powders of manganese were formed by the radiolytic reduction of permanganate (MnO₄^-)  at ambient temperature and pressure in (i) acid (ii) neutral and (iii) alkali mediums. Gamma radiolysis generated brown/black precipitates of manganese oxides were analyzed by X-ray powder diffraction, Raman spectroscopy, XPS, AFM and SEM. The influence of oxygen on the nature of oxides formed was studied. Permanganic acid (HMnO4), at low concentrations (2.5mM)and irradiated to 123kGy, produced l-MnO₂ with an average crystallite size of 30nm whereas at high concentrations (8 mM)and absorbed dose of 300 kGy, MnOOH was formed. The XRD pattern of the oxide powder obtained in all other cases showed broad peaks indicating nano-crystalline nature. The results of nitrogen adsorption–desorption experiments showed high specific surface area of ~53 m2/g corresponding to ~5 nm average pore size for the oxide obtained from 8 mM HMnO₄ irradiated to 300kGy. Atomic force microscopy (AFM) indicated a minimum particle size of ~13 nm for the same oxide whereas the SEM images showed presence of nano-rods.