Transition metal oxides are a class of materials that are vitally important for developing new materials with functionality and smartness. The unique properties of these materials are related to the presence of elements with mixed valences. The measurement of cation valence states can be performed using X-ray photoelectron spectroscopy and other chemical techniques, but these techniques are suitable only for a large quantity of specimens and the measured results are an average over all of the surface layer or the entire volume. For today’s research in the nanoera, it is important to determine the valence states from a region as small as a nanoparticle. In this Letter, we report the success of using electron energy-loss spectroscopy, attached to a transmission electron microscope, to measure the valence states in the Co-O and Mn-O systems at a spatial resolution of 20-70 nm. The reliability and sensitivity of the technique are demonstrated in reference to the composition curves and the electron diffraction data recorded in-situ during the reductions of Co3O4 and MnO2 and during the phase transformation in a solid solution of Fe2O3 and Mn2O3 as well.