The relationship between crystallization and growth of colloidal iron oxide nanoparticles during isothermal annealing was addressed in this work. The structural, morphological and chemical modifications of the nanoparticles during thermal treatments were followed by combination of electron microscopy, X-ray diffraction and spectroscopic methods. The initially monodisperse spherical nanoparticles with amorphous and partially oxidized structure evolved during the treatments, depending on the temperature and treatment time. Core-void-shell nanoparticles or single crystal nanoparticles and hollow polycrystalline nanoparticles, both with well defined Fe3O4 oxide phase, are formed depending on the conditions. This evolution was interpreted as a result of the Kirkendall effect associated to mass redistribution and fragmentation of the nanoparticles, bringing new information about the effect of post-synthesis treatments on the crystallinity and morphology of colloidal nanoparticles. (C) 2009 Elsevier Inc. All rights reserved.