Hydrothermal processing facilitates the synthesis of crystalline ceramic materials of varying composition and metastable phases. However, in most cases the relationships between processing parameters and the stoichiometry or solid solubility of hydrothermally derived materials are neither documented nor understood. This report summarizes a systematic evaluation of the effects of processing parameters on the stoichiometry and solid solubility of hydrothermally derived Ba_xSr_1-xTi0₃ (BST) powders and thin films. BST powders and thin films were produced by reacting either TiO₂ powder or titanium metalorganic precursors in alkaline aqueous solutions containing Ba and Sr. Processing variables included the solution pH, reaction time, reaction temperature, and the ratios of Ba, Sr, and Ti initially in the reaction solution. In order to provide bulk sampling for phase identification and quantitative compositional analysis, detailed experimental evaluations concentrated on BST powders. The effects of processing parameters on BST thin films were anticipated from BST powder results and experimentally verified. Thermodynamic explanation and prediction of the experimentally observed behavior was considered by modeling the equilibria of dissolved and precipitated ionic species in aqueous solutions. The experimental and thermodynamic assessments have established guidelines and elucidated important mechanisms for stoichiometry control and phase selection during hydrothermal synthesis of BST powders and films.