The gel chemistry of germanium is explored through the formation and composition of a hydrous metal oxide precursor gel used in the preparation of the HfGeO4 and HfGeO4:Ti X-ray phosphors. The enhanced solubility of hexagonal GeO2 in dilute ammoniacal solutions is exploited to give a convenient and high-yield precipitation. The precursor gel is shown by FT-IR to be a diphasic mixture of hydrous hafnia and an ammonium germanate gel. Thermal treatment drives the crystallization of a hafnium-rich, simple tetragonal Hf1-xGexO2 structure at 893 degrees C, that upon further heating to 1200 degrees C yields scheelite HfGeO4.