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The widely favored option for the safe storage of high-level radioactive waste is a multi-barrier disposal system with clay-based geotechnical barriers e.g. bentonite. Montmorillonite as a major constituent of bentonite (up to 90 wt.-%) is an important sink for potentially released radionuclides. The adsorption of 226Ra and Ba on Na-montmorillonite (SWy-2) was investigated as function of pH (edges) and of Ba concentration (isotherms) at different ionic strengths. Ba is often used as a chemical analogue for 226Ra due to their closely related physical and chemical properties e.g. the formation of complete Ba/Ra solid solutions. The experimental Ba/Ra adsorption data could be quantitatively described by considering cation exchange reactions on the planar sites of montmorillonite. To describe the pH dependent adsorption of Ba and Ra at high ionic strength and high pH, an additional reaction involving surface complexation was necessary. The fitted selectivity coefficients (Kc) for the cation exchange of Ba and Ra with respect to Na are very similar at low ionic strength (0.02 M), whereas at higher ionic strength (0.3 M), Ra appears to be more selective.