A phase transformation of micron-sized TiO2 powder from anatase to rutile was attempted by heat-treatment in order to generate a new mixed crystal TiO2 with high associated photocatalytic activity. Heat-treated micron-sized TiO2 powders at different transition stages were characterized by X-ray diffiraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM) methods. The tests of photocatalytic activity of the heat-treated micron-sized TiO2 powders were conducted by the photocatalytic degradation of Rhodamine B and Acid Red B under visible light irradiation. The results indicate that mixed crystal TiO2 photocatalyst heat-treated at 400 degrees C for 60 min shows the highest photocatalytic activity. It can effectively decompose the Rhodamine B and Acid Red B in aqueous solution after 6 h visible light irradiation. A remarkable improvement in photocatalytic activity of TiO2 is caused by the formation of combined rutile-anatase phases and separation of photogenerated electron-hole pairs. (c) 2007 Society of Chemical Industry.