Regarding investigations of structural changes in corrosion experiments, scanning electron microscopy and transmission electron microscopy (TEM) are imperative because of their ability to identify the composition and crystalline type (atomic structures) with a spatial resolution down to the nanometer order. Heretofore, energy-dispersive X-ray spectroscopy, selected area electron diffraction and direct imaging of atomic structures have been frequently employed for detecting compositions, determining crystalline, and visualizing (interface) atomic structures, respectively. However, literature on the use of electron energy-loss spectroscopy (EELS), combined with TEM and scanning TEM (STEM), in the study of EBCs, is scarce. Based on core-loss spectra obtained from EELS measurements, we can obtain compositional mapping and information on local electronic states (e.g., oxidation and spin states) . As the latter information is closely related to the coordination environments of each atom, this characterization technique is applicable to investigate the evolution of lattice defects, such as oxygen vacancies and GBs. Such state-of-the-art analyses of atomic and electronic structures are highly developed and have been broadly applied . Recently, Sternlicht et al. reported core-loss EELS analyses of Yb silicate polycrystals for investigating an interaction with CMAS glasses, wherein changes in compositions and crystalline structures have been detected . These analyses are expected to be useful also in the case of water vapor corrosion. For example, within the interior region of a Yb 2 Si 2 O 7 wafer, the number of Yb 2 SiO 5 particles increases and the size of these particles enlarges during water vapor exposure under oxygen potential gradients at 1673 K . This reaction is not caused by the decomposition of Yb 2 Si 2 O 7 to Yb 2 SiO 5 and SiO 2 but should relate to the creation and diffusion of cations and anions at around GBs. Core-loss EELS of such samples at the nanometer scale would facilitate an understanding of the local chemical environments associated with the defects involved and the detailed reactions that occur under high-temperature water vapor environments.