The safety margins of operating light water nuclear reactors in the United States are being enhanced through the development of accident tolerant claddings (ATCs), potentially avoiding severe core degradation under a beyond-design basis accident (BDBA) such as the Fukushima Daiichi accident of 2011. Under BDBA conditions, current zirconium alloy claddings have been observed to fail, their oxidized surfaces become non-passivating and Zr within the cladding be-coming self-catalytic, engendering the complete oxidation of the cladding and hydrogen release resulting in explosions. Developing alternative materials, such as advanced alloys with superior oxidation resistance, can mitigate against such a catastrophe. However, for success the composition and structure, such as morphology and extent of oxidation, of the ATC oxidation growth under BDBA conditions must be fully understood. Therefore, the aim of this investigation was to conduct multimodal analysis, employing microscopy, Raman spectroscopy, and synchrotron x-ray diffraction, to characterize the composition and physical properties of oxidation on a number of advanced alloys after exposure to air and steam at 1200°C for 2 h. These conditions simulate reactor temperature and atmosphere at the onset of a BDBA scenario, a novel time period for studying the oxidation mechanisms of advanced alloys given that a majority of studies focus on long-term exposure, usually∼10 to 100 h. Microscopy techniques elucidated the elemental composition of the oxidized region, how elements are spatially distributed throughout the region, and estimated of the extent of oxidation. Raman spectroscopy and synchrotron x-ray diffraction techniques con-firmed the stoichiometry of the oxide compounds that microscopy alluded to and corresponding structural phases of those compounds. This investigation thoroughly characterized four alloys: 1) ferritic Fe-Cr-Al-Mo alloy Kanthal APMT, 2) ferritic Cr-Mo-V steel T91, 3) Ni-Cr solid solution strengthened alloy Inconel 600 (A600), and 4) austenitic Cr-Ni alloy stainless steel 304 (SS304). Kanthal APMT develops a uniform, compact α-Al2O3 passivating layer in both air and steam that is ∼1μm thick. Steam exposure of T91 resulted in growth∼1000μm thick and featuring a rough morphology comprised of highly-faceted grains of α-Fe2O3 and Fe3O4, residing near the surface, followed by a porous network of primarily FeO, Fe(3–x)CrxO4, and α-Fe2O3 that extends subsurface. The oxidized surface of T91 after exposure to air is extremely porous, traversing about100μm into the alloy, with FeO, α-Fe2O3, and Fe3O4residing primarily at the surface, while Cr2O3 and Fe(3–x)CrxO4being observed predominantly sub-surface. Alloy A600 after exposure to steam generates an uniform, compact oxidation layer∼10μm, comprised of predominantly Fe(3–x)CrxO4,Fe3O4, and Cr2O3. After exposure to air, A600 generates an oxidation layer that is also∼10μm, featuring a rough surface with highly-faceted grains primarily comprised of NiFe2O4, with a compact sub-surface layer of predominantly Cr2O3and Fe(3–x)CrxO4. After exposure to air, SS304 generates a compact oxidized surface that is ∼10μm thick with sparsely-distributed pores, an outer layer enriched with Fe- and Mn-based oxidation products, including Fe3O4, Fe(NiMn)O4, and Mn2NiO4, followed by a Cr2O3-enriched and finally SiO2 sub-surface layers. In contrast, steam-oxidized SS304 is comprised by a∼550μm thick layer featuring a coarse-grained, Fe2O3-enriched outer layer featuring large voids, followed by a matrix of unreacted alloy alongside spinel oxides, Fe3O4 and Fe(3–x)CrxO4, and possibly Si-based oxides, SiO2 and Fe(3–x)SixO4. The results of this study are useful for understanding how the oxide structure and composition aid in the ability of ATC alloys to withstand extreme conditions. This research fills an important knowledge gap and offers a powerful tool aiding the design of the next generation of ATCs and provides insight into designing advanced alloys for similar applications.