The hydrothermal phase transition of ammonium titanate into TiO2 in aqueous ammonia media was investigated in the presence of F- ions by adding various kinds of electrolytes such as NaCl, NH4Cl, NaF, and NH4F. The phase composition and the morphologies of the products were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). With the assistance of F- ions, pure brookite TiO2 can be hydrothermally prepared with a wide range of Na+ ion concentrations from 0.25 M to 1.5 M in just 24 h, which has obvious advantages over the previous NaCl scheme [1,2]. Moreover, photocatalytic degradation of MO shows that both anatase and brookite prepared in the presence of F- ions show superior photocatalytic activities than those prepared in the absence of F- ions. The phase transition mechanism from ammonium titanate into TiO2 was also proposed. Besides the introduction of Na+ ions that benefits the phase transition from titanate to brookite, F- ions are suggested to play two key roles during the hydrothermal process: catalyzing the condensation reaction of Ti-OH to produce Ti-O-Ti linkages by forming Ti-F and regulating the intercalation and the de-intercalation of Na+ ions by counteracting the electrostatic interaction between the Na+ ions and the titanate layers. (C) 2012 Elsevier B.V. All rights reserved.