Nanocrystalline Fe-based spinels with composition Fe3-xTixO4 are synthesized using soft chemistry. Two steps are involved: precipitation in an aqueous solution followed by thermal annealing under a reducing mixture of N-2/H-2/H2O gases. Fe-segregation is found inside stoichiometric particles when the powders are studied ex situ; they exhibit a strong surface iron enrichment. This heterogeneity is related to kinetic effects linked to the difference of mobility between Fe2+ and Ti4+ cations during the partial oxidation of cations occurring ex situ. Stresses in the grains induced by oxidation govern the oxidation kinetics and lead to an abrupt compositional variation inside each particle. These heterogeneities in stoichiometric powders have been investigated by a combination of averaging and local techniques: XRD and Mossbauer spectrometry for an average analysis of powders, XPS for an analysis of the surface of the grains, and HRTEM for a local analysis of single grains.