| Summary: | <sup>226</sup>Ra is an important contributor to naturally occurring radioactive materials (NORM) and also considered in safety cases related to the disposal of spent nuclear fuel in a deep geological repository. Recrystallization and solid solution formation with sulfates is regarded as an important retention mechanism for <sup>226</sup>Ra. In natural systems sulfates often occur as (Ba,Sr)SO<sub>4</sub>. Therefore, we have chosen this solid solution at the Ba-rich end for investigations of the <sup>226</sup>Ra uptake. The resulting <sup>226</sup>Ra-solubility in aqueous solution was assessed in comparison with a thermodynamic model of the solid solution-aqueous solution system (Ba,Sr,Ra)SO<sub>4</sub> + H<sub>2</sub>O. The temperature and composition of the initial (Ba,Sr)SO<sub>4</sub> solid solution were varied. Measurements of the solution composition were combined with microscopic observations of the solid and thermodynamic modeling. A complex recrystallization behavior of the solid was observed, including the dissolution of significant amounts of the solid and formation of metastable phases. The re-equilibration of Ba-rich (Ba,Sr)SO<sub>4</sub> to (Ba,Sr,Ra)SO<sub>4</sub> leads to a major reconstruction of the solid. Already trace amounts of Sr in the solid solution can have a significant impact on the <sup>226</sup>Ra solubility, depending on the temperature. The experimental findings confirm the thermodynamic model, although not all solids reached equilibrium with respect to all cations.
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