Phase relations in the BaSO₄–RaSO₄–H₂O system are important for understanding the role of barite-type minerals in controlling the concentration of Ra²⁺ in natural water reservoirs. These relations are extremely sensitive to the difference in the solubility products of the end-members and to the degree of non-ideality of the solid solution phase. Experimental constraints to the standard entropy of RaSO₄ and the regular interaction parameter of the barite–RaSO₄ solid solution are ambiguous. This study is focused on determination of these parameters from first principles. The phonon density of states of RaSO₄ is computed with the aid of the density functional perturbation theory. The regular interaction parameter in the BaSO₄–RaSO₄ solid solution, W_BaRa, is interpreted as the slope of the enthalpy of mixing in the limit of infinite dilution (x_Ra = 0) and is calculated from the change in the total energy of a 2 × 2 × 2 supercell of BaSO₄ due to the insertion of a single substitutional defect of Ra. The method is validated by computing W values for a wider range of binary solid solutions with barite and aragonite structures. The computed value of W_BaRa = 2.50 ± 1.00 kJ/mol implies that the solid–aqueous equilibrium in the BaSO₄–RaSO₄–H₂O system may have an alyotropic point in close proximity to the BaSO₄ end-member. The assessment of available data on re-crystallization of barite in Ra-bearing aqueous solutions suggests that the barite crystals may fully equilibrate on the time scale of hundred days.