Abstract <p>The presence of colloids in groundwater can significantly complicate the behavior of radionuclides under the conditions of deep geological repository (DGR) of radioactive waste. To ensure the safety of radioactive waste disposal in the host rock environment, it is necessary to thoroughly study the properties of the colloids formed and determine their influence on the retention of radionuclides by crystalline rocks. This work investigates the influence of colloidal particles of bentonite from the Tagansky deposit on the sorption of ¹³⁷Cs, ⁹⁰Sr, ²⁴¹Am, ²³⁹Pu, and ²³⁷Np on samples of the host rock from the Yeniseisky site. Kinetic dependences were obtained, and the quantitative parameters of sorption of the above-listed radionuclides in a model groundwater solution on the colloidal fraction of bentonite from the Tagansky deposit and on intact rock samples were determined. A comparison of the sorption capacity of clay colloids and host rocks showed that the host rock makes a major contribution to the sorption of cesium and strontium, whereas plutonium and neptunium are predominantly retained by the colloidal fraction. The formation of even a small amount of colloidal material from engineered safety barrier materials influences the behavior of radionuclides under DGR conditions.</p>