Toxic disinfection byproducts such as trihalomethanes (CHCl3) are frequently found after chlorination for drinking water. Nano TiO2 which has been widely used for photocatalytic degradation of organic pollutants in wastewater, however, has relatively low effectiveness in the treatments of trace CHCl3. To engineer capable TiO2 photocatalysts, an understanding of their photoactive sites is of great importance and interest. By in situ X-ray absorption near edge structure (XANES) spectroscopy, photoactive sites such as A1 (4969 eV), A2 (4971 eV) and A3 (4972 eV) can be distinguished asfour-, five-, and six- coordinated Ti species, respectively in the nano-TiO2 (8.5 and 4.6 nm for TiO2 on SBA-15), TiO2 clusters (TiO2-SiO2), and highly atomic dispersed Ti (Ti-MCM-41) photocatalysts. It appears that the reactivity for the photocatalytic degradation of trace CHCl3 in drinking water lacks an expected relationship with the crystalline phase, band gap absorption edge, nor the particle size of the TiO2-based photocatalysts. Notably, the A2 sites being the main photocatalytic active species of the TiO2 may be accountable for the main (about 95%) photocatalytic degradation of trace CHCl3 in drinking water (7.2 ppm CHCl3/gTiO2∙hr). This work reveals that the A2 active sites of a TiO2-based photocatalyst are responsible for the photocatalytic reactivity, especially in photocatalytic degradation of CHCl3 in drinking water.