<p>Decontamination of ¹³⁷Cs from Fukushima soil is a very important issue after Fukushima Daiichi Nuclear Power Plant accident. Since most ¹³⁷Cs is strongly fixed to the clay minerals in the topsoil, the presence of soil surface contamination further increases the risk of long-term human exposure. Cationic surfactant can be used to desorb ¹³⁷Cs from clay minerals to water phase, and their decontamination efficiency depends on the clay mineral type. However, ¹³⁷Cs adsorbed to Fukushima soil is difficult to be decontaminated. One of the reasons is the aging effect, which causes the stronger retention of adsorbed ¹³⁷Cs over time. In order to investigate the aging effect on the decontamination efficiency from clay minerals contaminated with ¹³⁷Cs, different types of clays (Na- and Ca-bentonite; both predominantly composed of montmorillonite clay mineral) and clay minerals (illite and kaolinite) that had adsorbed ¹³⁷Cs were stored from 0 to 60&#xa0;days at room temperature, and then they were decontaminated using two types of cationic surfactants (benzyldodecyldimethylammonium bromide (BDAB) and dodecyltrimethylammonium bromide (DTAB)). No change in decontamination efficiency over storage time was observed for kaolinite and Ca-bentonite, while the change was observed for illite and Na-bentonite after 30&#xa0;days storage. The decontamination using DTAB tended to show more significant storage-time effect (aging effect) than BDAB, which can be attributed to the lower decontamination efficiency of DTAB. The aging effect tends to appear when the layer charge is more negative. This means that the hydrated Cs⁺ in the clay mineral (Cs⁺_aq/clay) is more easily dehydrated and adsorbed to the clay mineral having more negative charge to form inner sphere complex. The differences in aging effect were discussed from the point of the equilibrium between Cs⁺_aq/clay and Cs⁺_adsorption(inner sphere complex) + aq in the layer of clay mineral. It was suggested that there is a difference in the cause between illite and Na-bentonite. Cs⁺_aq/clay may move to the wedge-shaped intermediate zone (frayed-edge site) to be adsorbed as the dehydrated Cs⁺ and then move to the deeper place for illite by exchanging with K⁺. On the other hand, smaller amounts of water adsorption when keeping at room temperature for Na-bentonite compared to Ca-bentonite hinder the equilibrium movement to the hydrated Cs complexes (Cs⁺_aq/clay) direction. Both changes would shift the equilibrium to the dehydrated Cs from hydrated Cs in the clay mineral.</p>