Although cesium adsorbs onto the interlayer of clay minerals, coexisting natural organic matter (NOM) can inhibit this adsorption because of their blocking effect. A large amount of dissolved organic carbon (DOC) in the Pripyat River (= 18 mg L-1) is responsible for the large dissolved fraction of cesium in the river as suggested by cesium L-III-edge EXAFS. In addition, the carbon K-edge NEXAFS by STXM showed that humic acid coats clay minerals in the Pripyat River, which is responsible for the blocking effect.