The applications of nanoparticles are, to a large degree, limited by their size distribution. However, monodisperse nanoparticles are not always easily synthesised. In this scenario, size sorting is usually needed to separate the populations in differently sized colloidal nanoparticles. In this study we compare the effectiveness of three different size sorting methods: ultracentrifugation, ultrafiltration and magnetic separation, to sort dispersions of highly polydisperse ultrasmall iron oxide nanoparticles. After the sorting process, the polydispersity is significantly reduced and the different components obtained are analysed by TEM, XRD, DLS and magnetic measurements to shed new light on the composition of the aggregates of each fraction. We demonstrate that, at the USPIO level, the ultrafiltration gives better results than other methods and that the aggregates are formed by the largest nanoparticles. (C) 2013 Elsevier Ltd. All rights reserved.