In this work, manganese oxide nanoparticles were radiolytically synthesized in the presence or absence of diethylaminoethyl-dextran hydrochloride (DEAE-dextran). Deoxygenated potassium permanganate (KMnO4) alkaline aqueous suspensions were used as a precursor. The dose rate was similar to 32 kGy h(-1) and the absorbed doses were 100 and 200 kGy. The XRD patterns showed broad peaks indicating the low crystallinity and/or amorphous character of synthesized manganese oxides samples. The radiolytically synthesized samples in the presence of DEAE-dextran contained a mixture of phases, namely Mn2O3, Mn3O4 and an amorphous phase. The samples radiolytically synthesized with no added polymer consist of K0.27MnO2 center dot 0.54 H2O and Mn3O4 (hausmannite). The volume average domain size of hausmannite was estimated to similar to 45 nm using Scherrer equation and Williamson-Hall plots. The hydrodynamic diameters and zeta potential of the samples were measured. The pore radius distributions and pore volume of the obtained manganese oxides were determined by N-2 adsorption-desorption measurements. The synthesized manganese oxide samples were applied for decolorization of methylene blue (MB) aqueous solutions at pH = 2. The MB concentrations in the supernatant solutions were determined through the measurements of the UV-Vis absorbance intensities at a wavelength of 663 nm. The radiolytically synthesized Mn3O4 NPs in the absence of DEAE-dextran polymer (zeta potential of -39.4 mV, BET surface area of 234 m(2) g(-1) and pore volume of 1.52 cm(3) g(-1)) showed the highest MB decolorization effect.