Degradation of methylene blue (MB) is investigated using Mn3O4 as an oxidant. Discrete Mn3O4 particles and Mn3O4/Fe3O4 nanocomposites are synthesized via coprecipitation by using air as an oxidant in the absence and presence of previously synthesized magnetite nanoparticles, respectively. Characterization results show that superparamagnetic magnetite nanoparticles of 10 nm were synthesized. The Mn3O4 synthesis in the presence of these magnetite nanoparticles favors the formation of a pure, highly ordered Mn3O4/Fe3O4 superparamagnetic nanocomposite, which can be separated by an external magnetic field. This nanocomposite is applied to the oxidative decolorization of methylene blue (MB) and is capable of catalyzing the complete N-demethylation of MB, forming thionine as the final product and removing 93% of the dye in approximately 1 h. (C) 2013 Elsevier Ltd. All rights reserved.