The species of a series of F–TiO 2 support containing various amounts of fluorine (0–4.30wt.%) 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 NH 4 F. The state of the fluoride depends on the fluorine content. With lower fluorine content (≤0.94wt.%) only one kind of fluoride is formed. Further increase in the fluorine content (≥2.07wt.%) results in the formation of two kinds of fluorides. ESR analysis confirmed the formation of Ti 3+ species after fluorine-modification, which actually exists as Ti 4+ 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.