Nanostructured TiO2 with different crystal sizes (i.e., 3, 5, and 6 nm) but identical aggregate size (i.e., 100 nm) and shape (sphere) were successfully made using a modified sol-gel method. TiO2 catalysts with crystal size of 5 nm and aggregate size of 100 nm were also prepared. This batch of catalysts has similar phase structure, surface area, band-gap energy, and degree of surface hydroxylation, and differs mainly in their crystallinity. Synchrotron XRD, XANES and EXAFS, Raman spectroscopy and transmission electron microscopy (TEM) were used to characterize the structural properties of the nanometer sized TiO2, whereas ultraviolet, X-ray photoelectron and electron paramagnetic resonance spectroscopy provided important information on the electronic properties of these nanoparticles. The catalytic performance of the nanostructured TiO2 for gas-phase photocatalytic oxidation of trichloroethylene exhibited strong dependency on both crystal size and crystallinity.