Titanate nanofibers of various sizes and layered structure were prepared from inorganic titanium compounds by hydrothermal reactions. These fibers are different from "refractory" mineral substances because of their dimension, morphology, and significant large ratio of surface to volume, and, surprisingly, they are highly reactive. We found, for the first time, that phase transitions from the titanate nanostructures to TiO2 polymorphs take place readily in simple wet-chemical processes at temperatures close to ambient temperature. In acidic aqueous dispersions, the fibers transform to anatase and rutile nanoparticles, respectively, but via different mechanisms. The titanate fibers prepared at lower hydrothermal temperatures transform to TiO2 polymorphs at correspondingly lower temperatures because they are thinner, possess a larger surface area and more defects, and possess a less rigid crystal structure, resulting in lower stability. The transformations are reversible: in this case, the obtained TiO2 nanocrystals reacted with concentrate NaOH solution, yielding hollow titanate nanotubes. Consequently, there are reversible transformation pathways for transitions between the titanates and the titanium dioxide polymorphs, via wet-chemical reactions at moderate temperatures. The significance of these findings arises because such transitions can be engineered to produce numerous delicate nanostructures under moderate conditions. To demonstrate the commercial application potential of these processes, we also report titanate and TiO2 nanostructures synthesized directly from rutile minerals and industrial-grade rutiles by a new scheme of hydrometallurgical reactions.