[en] Highlights: • Bentonite heated forms suffered some structural changes upon thermal treatment. • Heated bentonites at 150 & 300 are preferable for ¹³⁷Cs sorption in salt media. • Heated bentonite at 600 °C is recommended for ⁸⁵Sr sorption. • Ion exchange with Ca²⁺ and Mg²⁺ is more effective for ¹³⁷Cs uptake than OH⁻ groups. • ⁸⁵Sr sorption reflects chemical bonding with OH⁻ or coprecipitation of Sr(OH)₂. - Abstract: This study focuses on the structural changes of bentonite upon thermal treatment at various temperatures, i.e. 150, 300 and 600, and their effect on the sorption behaviour for ¹³⁷Cs and ⁸⁵Sr. Thermal treatment caused gradual disappearance of the major peak of montmorillonite (at 15.15 Å) and minimization of calcite as observed by powder x-ray diffraction, in addition thermal gravimetric analysis indicated the occurrence of dehydration and dehydroxylation of the materials upon heating. Sorption experiments were carried out by batch-method and data were expressed in terms of distribution coefficients (K_d). Contact time, solution-solid ratio and radionuclide activity concentration were investigated in order to determine the optimal conditions and speculate the mechanism of the uptake. Sorption data revealed that the heated forms had lower selectivity for ¹³⁷Cs than the original bentonite, while B-150 and B-300 were the best sorbents when ¹³⁷Cs presents in salt media. In either case, ion exchange process of ¹³⁷Cs with the crystal lattice cations (i.e. Ca²⁺ and Mg²⁺) was the governing mechanism. Contrarily, bentonite heated forms are recommended sorbents for ⁸⁵Sr, especially B-600. The outperformance of thermally treated bentonites reflected chemical bonding reaction with the surface OH groups as well as precipitation of Sr(OH)₂.