Conditions for a sulfate method for the synthesis of a metastable modification, which has been previously described as “η-TiO₂” (Dadachov, 2006), were found and the stability region of this phase in the hydrolysis temperature–hydrolysis duration coordinates was determined. Investigation by a number of methods (X-ray powder diffraction, a differential dissolution method, thermogravimetry, IR spectroscopy, Raman spectroscopy) showed that the η-phase is not a polymorph of TiO₂ but is a pseudo-polymorph of titanium dioxide hydrate. It was demonstrated that nanoparticles of the low-temperature η-phase consist of the [TiO_2−x·mH₂O] core, the structure of which can be described as a superstructure in relation to anatase, and an amorphous shell containing TiO_2−x (trace amount), OH, HSO₄ and water. The average crystallite size depends on the ratio of the constituents.