Real-time-resolved observation by using X-ray absorption fine structure spectroscopy was employed in order to study the interaction between dropped water and a mixture of clay minerals and cesium chloride. Dissolution of cesium chloride in water and adsorption of cesium ions on clay minerals were observed under in situ condition and real-time-resolved mode. Change of X-ray absorption spectra were used for the evaluation of amount and chemical state of cesium compounds. For sericite and illite clays, relatively long lifetime of hydrated cesium ions was observed before adsorption on clay minerals. On the other hand, for vermiculite, kaolinite and zeolite clays, hydrated cesium ions were not intensely observed due to short reaction time between hydrated cesium ions and the clay minerals. The electronic configuration of cesium adsorbed on the vermiculite clay is different from that of cesium adsorbed on the other clay minerals. Real-time-resolved X-ray absorption fine structure spectroscopy observation is expected to be widely used for the study of the cesium adsorption on many clay minerals.