Pure samples of (NH4)(2)V6O16 were prepared by hydrothermal crystallization of amorphous ammonium vanadate hydrate in the presence of LiOH solution. By changing the conditions of the hydrothermal treatment, crystallites differing by one order in size resulted. The ammonium hexavanadate thus prepared was thermally investigated by simultaneous TG and DTA, as a function of heating rate under a constant stream of different gases.