The use of bentonites as soil amendment has met with little success in reducing plant uptake of radiocaesium. However, bentonites exchanged with K+ have pronounced Cs+ binding capacity when subjected to wetting–drying cycles. Fifty-four different bentonites were collected and characterized for cation exchange capacity and chemical composition. The radiocaesium interception potential (RIP) increased up to 160-fold (mean 25) when the bentonites were converted to the K-form and subjected to wetting–drying cycles. This increase in radiocaesium sorption was ascribed to a collapse of the clay sheets into an illite-like structure, and was most pronounced in bentonites with a high layer charge. The RIP values of K-bentonites subjected to 25 wetting–drying cycles ranged from 0.22 to 44.3 mol kg−1. The RIP yields, i.e. the RIP in soil–bentonite mixtures expressed per unit bentonite added, were even higher and ranged up to 99 mol kg−1. This upper limit is about 10-fold higher than the RIP value of illite (∼ 10 mol kg−1), the principal 137Cs sorbent in soils of temperate climates. Wetting–drying also promoted fixation of radiocaesium in soils amended with K-bentonites. About 30% of added 137Cs could be desorbed with 1 m ammonium acetate (NH4Ac) from an unamended soil after 25 wetting–drying cycles, while only between 8 and 21% of 137Cs could be desorbed from a soil amended with bentonite and a K-salt. These findings support the proposition that addition of K-bentonite may be effective in reducing availability of 137Cs in soils.