The structure of TiO2 hydrates is studied by X-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The samples prepared by NH4OH precipitation from an aqueous TiCl4 solution are amorphous or partially crystalline. They consist of a mixture of distorted anatase and brookite crystallites, each having a diameter of 20 nm. The sample prepared by thermal hydrolysis from a titanium oxide sulfate solution was totally crystalline, composed only of distorted anatase crystallites with a diameter of 15 nm. Primary particles can be oriented in chainlike structures. The crystallinity and the crystal shape reveal the sample history. The applicability of the AFM and TEM measurements for structural investigations of the powder specimens concerned is briefly discussed.