Effects of vanadium on the desulfurization performance of FCC catalysts were investigated with vanadium having different oxidation numbers (abbr. Oxnum). Molecular modeling studies showed that vanadium with low Oxnum could affect the chemical conversion of sulfur compounds. However, the vanadium deposited on equilibrium catalyst was in high Oxnum because of the oxidation in regenerator, so an activation method to reduce vanadium Oxnum named selective activation was introduced. It was proved with electron paramagnetic resonance (EPR) and temperature-programmed reduction (TPR) that vanadium Oxnum decreased when the catalyst was activated. The molecular modeling studies are well consistent with the lab evaluation results. The desulfurization performances of activated equilibrium catalysts were better than that of the unactivated catalysts. Similar results were observed with the lab vanadium-contaminated catalyst. The desulfurization performance of the catalyst was optimized when vanadium Oxnum was close to 2 (VO). (c) 2004 Elsevier B.V. All rights preserved.