The sorption of caesium and strontium onto clay minerals by exchange with other, charge compensating cations was studied by batch experiments. It was demonstrated that, for clay minerals with a net negative charge arising from isomorphous substitution within the crystal lattice, caesium and strontium sorption is predominantly a cation exchange process. Strontium exchange with charge compensating sodium ions on clay minerals, in the absence of competing cations, results in high strontium R3 values until exchange with sodium is virtually completed. Strontium exchange for calcium ions markedly reduces the strontium R4 under similar conditions. This is consistent with calculations to determine the selectivity coefficient for the exchange reaction. For the exchange of divalent strontium for monovalent sodium ions, theory predicts that the selectivity of the clay to the divalent cation increases with its loading. This is not the case for the homo-, divalent exchange of strontium for calcium and the marginal difference in hydration energies of the two cations further depresses strontium-selectivity of clays. Clay minerals are selective to caesium ions because of their low hydration energy and the Rd for caesium exchange with sodium is higher than for potassium exchange with sodium under similar conditions. Caesium sorption can cause 2: 1 swelling clay minerals to physically alter their structure as caesium loading increases. For this reason, caesium exchange on smectites does not fit Langmuir nor Freundlich plots. Also, the selectivity coefficient was found to vary widely with loading, compared with potassium exchange on smectites or caesium exchange on non-swelling minerals. Complete caesium exchange on Los Trancos montmorillonite causes the interlayer to collapse. Desorption of caesium ions from planar surface and edge sites can be achieved but the majority of caesium in the interlayer region is resistant to desorption and strong acid attack.