We first describe with the help of reference experiments (at the Fe2p and O1s edges) and ab initio calculations how electron energy loss spectroscopy (EELS) can be used in order to characterize phases of iron oxide and/or hydroxide nanomaterials. In particular we show that dehydration of iron hydroxides such as goethite can easily appear under the electron beam but might be followed by monitoring the OK peak. Indeed both local spin-density approximation (LSDA) and LSDA+U calculations confirm that intensity of the prepeak of OK should increase while H atoms are removed. We also demonstrate that different magnetic orders do not change significantly the OK EELS fine structure of goethite. Thus, nanomaterials (particles and wires) synthesized by a hydrothermal treatment of nanoscale (10-40 nm) magnetite particles have been conducted. Among them, crystalline iron oxide nanowires with average diameter of 20 nm and length of up to 10 μm are reported. The OK edge and Fe L 2,3 edges were studied by EELS for these nanostructures. The results indicated that the valence of iron is 3+ in the wires while it is the mixture of 2+ and 3+ in the particles. From these combined EELS, scanning transmission electron microscopy, diffraction, and high-resolution electron microscopy, the complexity of the produced phases from these hydrothermal treatments can be revealed. This work shows how EELS with high-energy resolution is a unique tool to differentiate iron oxide compounds such as the tricky magnetite-maghemite solid solution or the case of partially dehydrated phases, even on a nanometer scale. © 2009 The American Physical Society.