The extent of waterside corrosion of zirconium alloy fuel cladding is a strong function of the specific alloy used. To rationalize the details of the corrosion kinetics obtained during autoclave corrosion of alloys, a Transmission Electron Microscope (TEM) study of Zircaloy-4 oxide structures is performed at various stages of corrosion. Using focused ion beam (FIB), TEM samples were lifted out of the oxide layer at precise locations in the oxide, or near the oxide/metal interface. The study characterizes the oxide grain size and morphology, oxide texture, incorporation of metallic precipitates and the evolution of these with respect to the distance to the oxide/metal interface. Special attention was given to the oxide/metal interface to determine the phases present, and measure the evolution of oxygen content in the oxide precursor layers. Results show that the oxide is mainly monoclinic, with a small fraction of tetragonal phase, with a preponderance of columnar oxide grains, which extend to the oxide/metal interface. The interface exhibits both an intermediate layer and a discontinuous layer of "blocky" grains on the metal matrix side, which change upon transition and which could be identified with suboxide phases, corresponding to a level of higher oxygen content.