A route for the low temperature preparation of nickel manganite spinels is described. This method has allowed to obtain, for the first time, tetragonal nickel manganite spinels NixMn3-xO4, in the composition range 0.6 less-than-or-equal-to x less-than-or-equal-to 1.0. This phase has been prepared by heating at 200-degrees-C gels obtained by addition of 1M n-butylamine to mixed Ni2+ and Mn2+ solutions. In this paper, we report about the results obtained from X-ray studies, of the thermal evolution and reactivity for the tetragonal spinel with x = 0.8, between 200-degrees to 1000-degrees-C. The formation of a metastable gamma-cation deficient cubic spinel is attained at 300-degrees-C; it decomposes at almost-equal-to 500-degrees-C, giving at 600-degrees-C a mixture of NiMnO3 and a-Mn2O3. Above 800-degrees-C, the reaction between these two phases leads to the formation of the high temperature cubic spinel, in which (Mn4+/Mn3+)B almost-equal-to 1.0. Cell lattice and oxygen positional parameters have been determined from Rietveld refinements. From these data, ionic configuration and cation distribution formulae for cubic spinels have been derived. These results can account for the mechanism of transformations that takes place during the thermal treatments.