Titanium silicalite (TS-1) is used as catalyst in selective oxidation of thiophene by hydrogen peroxide at 333 K. Fourier transform infrared (FT-IR) and UV-vis characterization shows the presence of framework titanium in all TS-1 samples we prepared with different SiO2/TiO2 ratio. Catalytic test over these catalysts confirms that framework Ti species does influence the oxidation of thiophene. The role of non-framework titanium was discussed by studying acid treating of the catalyst. If TS-1 is treated using HCl (2 mol/l) four times, part of the non-framework titanium species will be washed off and the activity of the catalyst increases greatly. These results suggest that framework titanium was crucial for thiophene oxidation. Non-framework titanium is not the active site, but provides steric hindrance for this reaction. Obvious diffusive restriction on the thiophene oxidation was also observed when using TS-1 with different crystal size. (C) 2004 Elsevier B.V. All rights reserved.