Mixed titanium dioxide (TiO2) powders of various amorphous-anatase-rutile contents were prepared by acid-catalyzed sol-gel method at 80 degrees C without calcination. The physico-chemical properties of the powders were investigated by powder XRD, SEM, Brunauer-Emmett-Teller (BET), Fourier-transformed infrared (FT-IR), X-ray energy-dispersive (EDX), and UV-vis techniques. The results showed that the products were mixtures of mainly amorphous TiO2 with small amount of anatase and rutile phases when prepared by without using acid catalyst and by adding small amount of acid catalyst (hydrochloric acid, nitric acid, and acetic acid). However, when either sulfuric acid or phosphoric acid was used the products obtained were mainly amorphous TiO2 with small amount of anatase. Mechanism of growth from basic units to nanocrystalline was also proposed. Positive photocatalytic activities of these products were found and some could be compared favorably with Degussa P25. Three dyes, methylene blue, crystal violet, and congo red were used in the photocatalytic studies. (C) 2008 Elsevier B.V. All rights reserved.