The adsorption of 137Cs by sediments from the Savannah River Plant follows a theoretically expected linear relationship between ln Kd* and l/T where Kd* and T are the equilibrium distribution coefficient (Cs adsorbed, meg g−1/Cs+ in solution, meq ml−1) and the temperature in kelvins, respectively. The slope of these plots decreases after removal of hydroxy Al interlayers with citrate pretreatment. Hydroxy Al interlayers thus make Cs+ adsorption more temperature dependent. This thermal effect of hydroxy Al interlayers becomes more significant in sediments containing a nonswelling 2:1–2:2 intergrade clay mineral than in those containing a swelling 2:1–2:2 intergrade clay mineral. The phenomenon is explained by considering hydroxy Al interlayers as a source of hydronium ions H3O+ which compete with Cs+ for exchange positions in wedge zones. An increase in temperature favors an increase in H3O+ ions, which in turn favors less adsorption of Cs+. Aside from their thermal effect the positively charged hydroxy aluminum polymeric groups drastically decrease the cation exchange capacity and consequently the adsorption of cesium.