A novel and simple method for preparing highly photoactive nanocrystalline F⁻-doped TiO₂ photocatalyst with anatase and brookite phase was developed by hydrolysis of titanium tetraisopropoxide in a mixed NH₄F−H₂O solution. The prepared F⁻-doped TiO₂ powders were characterized by differential thermal analysis-thermogravimetry (DTA-TG), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV−vis absorption spectroscopy, photoluminescence spectra (PL), transmission electron microscopy (TEM), and BET surface areas. The photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air. The results showed that the crystallinity of anatase was improved upon F⁻ doping. Moreover, fluoride ions not only suppressed the formation of brookite phase but also prevented phase transition of anatase to rutile. The F⁻-doped TiO₂ samples exhibited stronger absorption in the UV−visible range with a red shift in the band gap transition. The photocatalytic activity of F⁻-doped TiO₂ powders prepared by this method exceeded that of Degussa P25 when the molar ratio of NH₄F to H₂O was kept in the range of 0.5−3. Copyright © 2002 American Chemical Society.