Powders of the microwave dielectric material barium magnesium tantalate Ba(Mg1/3Ta2/3)O-3 have been produced by hydrothermal synthesis at moderately low temperatures (160 to 350 degrees C). It was found that while it is relatively straightforward to produce the material in the desired perovskite phase at or below 200 degrees C, the powder particles tend to be highly irregular in morphology with extremely small dimensions (of the order of 10 nm) and deficient in magnesium (with some precipitation of the excess magnesium as the hydroxide). The effects of both higher synthesis temperatures and different feedstock preparation were thus investigated with the aim of improving the precipitation of magnesium under hydrothermal conditions in order to produce a more homogeneous, stoichiometric powder and significant progress was made. It was found that when near-stoichiometric particles are formed, they adopt rounded morphologies and exhibit larger particle sizes (around 30-50 nm). These results show that the hydrothermal feedstock and the synthesis temperature used have a profound effect on particle stoichiometry, which in turn affects the growth morphology of the particles.