The properties of manganese oxide nanomaterials are dictated by the structure and morphology of the particular phases they can adopt. In this work, the effects of post-synthesis heat treatments on amorphous monodisperse manganese oxide nanoparticles at temperatures of up to 1300 degrees C under laboratory air and argon atmospheres have been investigated using X-ray diffraction, thermal analysis, and electron microscopy techniques. During heat treatments in air, the nanoparticles undergo three transformations, resulting in: crystallization to cubic Mn2O3 at ~ 500 degrees C, followed by the transformation to tetragonal Mn3O4 at ~ 1010 degrees C, and to cubic Mn3O4 at ~ 1190 degrees C. The first two transformations are irreversible, are associated with oxygen loss, and involve reductions of the Mn ions. The latter transformation is polymorphic and spontaneously reversible, and so, tetragonal Mn3O4 is observed at ambient temperature in samples heat-treated at above 1000 degrees C. The samples heat-treated in argon firstly crystallize to a mixture of monoclinic Mn2O3 and tetragonal Mn3O4 at ~ 475 degrees C, followed by complete transformation to tetragonal Mn3O4 at ~ 820 degrees C, and then to mostly cubic MnO at ~ 1145 degrees C; here again, the residual tetragonal Mn3O4 undergoes a reversible polymorphic transformation at ~ 1180 degrees C, whereas the other transformations are irreversible. In both atmospheres, the amorphous material exhibits short-range cryptomelane-type order with a mixture of Mn3+ and Mn4+ prior to crystallization. These data indicate that most of the stable manganese oxide phases can be obtained from initially amorphous nanoparticles by heat treatment under appropriate conditions.