In this paper. the results of a study on the effects of catalyst properties and contaminant metals on FCC gasoline sulfur are reported. Data for four steamed deactivated FCC catalysts showed that the higher the unit cell size (UCS) of an FCC catalyst, the lower the Sulfur concentration in FCC gasoline. These results suggest a mechanism that explains gasoline sulfur reduction through a hydrogen transfer step resulting in saturation of thiophenes. The effects of contaminant metals, that usually exist oil FCC catalysts (Ni and V), on FCC gasoline sulfur, were examined for both equilibrium and deactivated catalysts. The presence of these metals lowers gasoline sulfur dramatically. Vanadium (V) in its highest oxidation step is most effective. It appears that these metals catalyze the cyclization of long alkyl-thiophenes to form alkyl-benzothiophenes boiling outside the gasoline range. Finally, it was observed that the presence of ZSM-5 additive resulted in an increase in gasoline sulfur. (C) 2003 Elsevier B.V. All rights reserved.