The process of adsorption of strontium ions on the modified layered aluminosilicate - montmorillonite was studied in the paper. The thermodynamic parameters of the adsorption were calculated. The investigations were carried out on the model solutions of stable strontium. By modifying montmorillonite surface it was established a significant increase of its adsorption capacity. The parameters of the Gibbs energy such as ΔG, ΔH, T × ΔS were determined. Received and investigated adsorbents can be used for the deactivation of water containing radionuclides Sr90.