The thermolysis of some transition metal ferrioxalate precursors, M3[Fe(ox)3]2·xH2O (M = Mn, Co, Ni, Cu) has been studied in a static air atmosphere from ambient to 600 °C. Various physicochemical techniques e.g. TG, DTG, DTA, XRD, IR, Mössbauer etc. have been employed to characterize the intermediates and products. After dehydration the anhydrous precursors undergo decomposition to yield an iron(II) intermediate, M[FeII(ox)2] (M = Mn, Co, Ni) in the temperature range 175–290 °C. A subsequent oxidative decomposition of the iron(II) species leads to the formation of α-Fe2O3 and MO in successive stages. Finally a solid state reaction occurs between the oxides above 400 °C resulting in the formation of transition metal ferrites, MFe2O4. The ferrites have been obtained at much lower temperature and in less time than in the conventional ceramic method.