The species of a series of F-TiO2 support containing various amounts of fluorine (0-4.30 wt.%) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and pyridine-adsorbed IR. The results show that the addition of fluorine has a large effect on titania support and two kinds of fluorides are found to be formed in titania support after titania support is treated by NH4F. The state of the fluoride depends on the fluorine content. With lower fluorine content (<= 0.94 wt.%) only one kind of fluoride is formed. Further increase in the fluorine content (2.07 wt.%) results in the formation of two kinds of fluorides. ESR analysis confirmed the formation of Ti3+ species after fluorine-modification, which actually exists as Ti4+ species containing an oxygen vacancy with an electron trapped in. Co-Mo catalysts supported on fluorine-modified titania presented higher HDS activities than Co-Mo catalyst supported on untreated titania. The hydrodesulfurization mechanism on sulfided fluorine-promoted cobalt-molybdenum/titania catalysts was discussed in this report. (c) 2006 Elsevier B.V. All rights reserved.