TiO2 nanoparticles were prepared using the hydrolysis of titanium tetraisopropoxide (TTIP) using TENOH as a peptizing agent in the hydrothermal method. The physical properties of prepared nanosized TiO2 particles were investigated. The photocatalytic degradation of orange 11 has been studied using a batch reactor in the presence of UV light. No rutile phase was shown even though the molar ratio of TENOH/TTIP increased up to 1.0. The crystallite size of the anatase phase is increased from 15 to 30nm as the molar ratio of TENOH/TTIP increases from 0.1 to 1.0. The titania particle prepared at TENOH/TTIP molar ratio = 0.1 shows the highest activity on the photocatalytic decomposition of orange 11 and the photocatalytic activity decreases according to an increase in TENOH/TTIP molar ratio. In addition, the titania particles prepared at 160 degrees C shows the highest activity on the photocatalytic decomposition of orange II. (c) 2007 Elsevier Ltd. All rights reserved.