Straight-run diesel fuel containing 1.6% of sulfur was enzymatically oxidized with chloroperoxidase from Caldariomyces fumago. Most organosulfides and thiophenes were transformed to form sulfoxides and sulfones. The oxidized organosulfur compounds can be effectively removed by distillation. The resulting fraction after distillation contained only 0.27% sulfur, while the untreated straight-run diesel fuel after the same distillation process still showed 1.27% sulfur. To know the chemical nature of the products, nine organosulfur compounds and 12 polycyclic aromatic compounds (PACs) were transformed by chloroperoxidase in the presence of chloride and hydrogen peroxide. Organosulfur compounds were only oxidized to form sulfoxides and sulfones, and no chlorinated derivatives were detected, except for bithiophene. in contrast, PACs were exclusively chlorinated, and no oxidized derivatives could be found. No enzymatic activity was detected on PACs with an ionization potential higher than 8.52 eV, while in the lower region it was found that the higher the ionization potential of the PAC the tower the specific activity. On the other hand, the substrate ionization potential did not seem to influence chloroperoxidase activity in the oxidation of organosulfur compounds. All organosulfur compounds tested were oxidized by chloroperoxidase. From double-substrate experiments, it appears that organosulfur compounds are oxidized by both compound I and compound X enzyme intermediates, while PACs react only with the halogenating intermediate, compound X.