[en] This study aimed to reveal local structures of strontium (Sr) and cesium (Cs) adsorbed on clay minerals (vermiculite and montmorillonite), which suggests stabilities of the adsorbed species of ⁹⁰Sr and ¹³⁷Cs in the environment. Adsorption experiments to determine solid-water distribution, XRD for interlayer distances of the clay minerals, and EXAFS for speciation of Sr and Cs were conducted. They showed that Sr and Cs formed outer- and inner-sphere complexes, respectively, in the interlayer, which explains the different degree of mobility of ⁹⁰Sr and ¹³⁷Cs in the environment and indicates larger contribution of electrostatic effect for Sr²⁺ than for Cs⁺. (author)