Perovskite BaMg_1/3Ta_2/3O₃ sol-derived powders have been produced by hydrothermal synthesis at low temperatures of 160°C - 350°C. The particles in these powders tend to be irregular shaped, ultra-fine and magnesium-deficient. Methods have been found for producing near-stoichiometric particles and these have more rounded morphologies and larger sizes. Thus, a clear link has been established between the stoichiometry and morphology of the powder particles which suggests that the particle growth mechanisms under hydrothermal conditions are affected strongly by stoichiometry.