Fibrous titania samples consisting of monoclinic TiO2, anatase, and rutile were synthesized by heat treatment of H2Ti4O9. nH(2)O, which was prepared by the ion-exchange reaction of potassium tetratitanate in HCl, in different supercritical media. The phase transformation temperature and microstructure of the products changed significantly depending on the heating environment. The critical temperature at which anatase appeared in liquid media was much lower than that at which it appeared in air, indicating that the phase transformation of the monoclinic TiO2 to anatase proceeded by the dissolution-precipitation mechanism. Fibrous titania consisting of nanosize crystals of TiO2 possessing high crystallinity could be obtained by treatment using supercritical ethanol or methanol as the reaction medium. The photocatalytic activities for hydrogen evolution from an aqueous methanol solution and for phenol photodegradation were determined for various titania samples. Amorphous TiO2, which possessed large amounts of crystal defects, showed no photocatalytic activity for either reaction. The titania that crystallized in supercritical ethanol or methanol consisted of a mixture of monoclinic TiO2 and anatase and showed excellent hydrogen evolution activity, and the hydrogen evolution activity of fibrous titania changed with heat treatment media in the following sequence: ethanol > methanol > water > 8-propanol > n-butanol > n,-hexane > P-25 > air. On the other hand, the phenol degradation activity of titania decreased with decreasing specific surface area as follows: P-25 > water > ethanol > methanol > n-butanol > air.