Cross-sectional observation of stainless steel reacted with cesium-hydroxide vapor (right figure) showed similar distributions of Si and Cs in the Cs-adsorbed layer. This suggests that Si and Cs can form compounds. Furthermore, the identification of chemical species by micro-HAXPES (lower-left figure) indicated that the chemical forms of Cs compounds depended upon the Si content in the SS (right figures). In the future, we will try to improve estimates of the Cs distribution within nuclear reactors by establishing a model that considers the chemical properties of the identified Cs compounds.