A thermodynamic model was developed to describe the stability of (Ba,Sr)TiO3 (BST) solid solutions in the Ba-Sr-Ti-K-(EDTA)-H2O (EDTA = ethylenediaminetetraacetic acid) system. Phase diagrams were computed to identify the range of conditions suitable for making phase-pure BST. Hydrothermal experiments were performed to validate the thermodynamic model. The model was found to be more useful when an ideal solid solution was used to estimate the energetics for the BST phase instead of experimental thermodynamic data. In addition, EDTA was found to promote stable conditions for BST formation. When attempting to prepare Ba0.50Sr0.50TiO3 without EDTA, BaTiO3-rich and SrTiO3-rich phases precipitated separately, at 70degrees-160degreesC. However, in the presence of EDTA, a phase-pure Ba0.55Sr0.45TiO3 solid solution was obtained at 90degrees-120degreesC. EDTA is effective because it prevents phase heterogeneities from forming and equalizes the adsorption affinity of strontium and barium species.