Scrolled nanotubes of TiO2 are found to contain a large fraction of undercoordinated surface sites that scroll layers of anatase titania in proton-deficient aqueous systems. Reversible tube-to-sheet and sheet-to-tube transitions, and associated reversible lattice transformations between quasianatase and quasirutile, are obtained, respectively, by protonation and deprotonation of undercoordinated surface sites (see Figure).