[en] ²²⁶Ra²⁺ and ¹³³Ba²⁺ uptake by barite in aqueous solution is studied on the basis of batch type experiments with two different barite powders with different specific surface area (0.5 m2/g and 3.2 m2/g, respectively). The uptake of ²²⁶Ra²⁺ and ¹³³Ba²⁺ is not only limited to adsorption reactions but proceeds significantly into the bulk of the barite crystals. ¹³³Ba²⁺ uptake kinetics is affected by various parameters, such as amount of sample, specific surface area, sample type and solution composition. In the case of ¹³³Ba²⁺, complete isotopic equilibration of the ¹³³Ba²⁺ spiked solution with the barite powder occurs within 50 to 600 days. This information is derived by monitoring the aqueous ¹³³Ba²⁺ concentration combined with simple mass balance calculations. In the case of ²²⁶Ra²⁺ a Ra_xBa_1-xSO₄ solid solution forms and the uptake rate drops significantly within 400 days. The observed ²²⁶Ra²⁺ concentration in solution is controlled by the solubility of a Ra_xBa_1-xSO₄ solid solution and several orders of magnitude below the Ra²⁺ solubility with respect to a pure RaSO₄(s) end member. It cannot be demonstrated unambiguously that a zero exchange rate and therefore thermodynamic equilibrium has been established within the observation period. The observed concentrations may be interpreted either as (1) a partial equilibration of 20 to 50% of the barite crystals with ²²⁶Ra²⁺ or (2) as complete equilibration of a Ra_0.000128Ba_0.999872SO₄ solid solution with ²²⁶Ra²⁺ with no pure barite left. In both cases it is concluded that equilibration between aqueous Ra²⁺ and barite involves the replacement of a substantial fraction of the initial barite and proceeds significantly beyond pure surface adsorption processes