Nanometric spinel oxide powders, Fe3-xMxO4, where M is a transition element, have been synthetized by soft chemistry. This method generally leads to a non-stoichiometric phase, Fe3-xMxO4+delta where delta is the deviation from stoichiometry so that further annealing at low temperatures around 450 degrees C and low oxygen partial pressure around 10(-25) Pa given by N-2/H-2/H2O gas mixtures is required : this enables a stoichiometric compound to be obtained and a nanometric size to be maintained. The complete set up consisting of a gas mixer, a thermogravimetric apparatus and a preparative furnace is described. Some results concerning the conditions of temperature and oxygen partial pressure leading to stoichiometry are given in the case of vanadium ferrites containing or without Co.