Nanostructured TiO2 with 11-(P11) and 5-nm (P5) crystal sizes were prepared by a modified sot-gel method. Controlled crystallization and a pretreatment process were employed to obtain TiO2 with different surface roughness and degree of hydroxylation, while maintaining an identical crystal (i.e., 11 or 5 nm) and aggregate (i.e., 100 nm) sizes, phase structure (i.e., anatase), and crystallinity (i.e., X-ray diffraction peak intensity). Using the photooxidation of airborne trichloroethylene as a probe reaction, we were able to identify that the hydroxyl groups on low-coordinated titanium atoms are responsible for the generation of dichloroethylene and dichloroacetaldehyde by-products. Their presence usually means lower TCE conversion and in some cases leads to catalyst deactivation. (C) 2003 Elsevier Inc. All rights reserved.