Nanophase TiO2 particles, with typical particle sizes in the range of 20-50 nm, have been synthesized using a microemulsion-mediated process. In this process, the aqueous cores of water/Triton X-100/hexanol/cyclohexane microemulsions have been used as constrained microreactors for the precipitation of precursor titanium hydroxide. The hydroxide particles thus formed were separated, dried, and calcined at different temperatures to form nanoparticles of TiO2. In order to see the phase transition temperature, thermogravimetric analysis/differential thermal analysis studies were performed on the precursor hydrodoxide particles. The average particle size of these particles was determined by transmission electron microscopy, BET surface area and line broadening by X-ray diffraction. Phase transformation of these particles was confirmed by X-ray diffraction. The attenuation of ultraviolet radiation increased as the particle size decreased. As a catalyst for the photodegradation of phenol, only the anatase form of TiO2 showed significant degradation of phenol, whereas the rutile form of TiO2 was totally inactive for this reaction.