Pure and Mn-doped willemite, Zn2SiO4 of different morphologies acid particle sizes were prepared with and without NH4OH under different hydrothermal conditions. Pure willemite was synthesized at a temperature as low as 165 degrees C using autogeneous pressure. The nature of the an ion, silica source and the NH4OH :Zn2+ ratio played an important role in the size and morphology of the willemite phase. The presence of SO42- anions resulted in mainly equiaxed grains. Such powders are expected to be good for consolidation and in the processing of phosphors.