Reductions in the maximum permissible sulfur content of diesel fuel to less than 0.05 wt % will require deep desulfurization technologies to meet these standards. In some refineries, new technologies for desulfurization and new hydrodesulfurization catalysts may be required for production of low-sulfur diesel fuel. Some of the major components of diesel fuel are dibenzothiophene (DBT) and alkyl-substituted dibenzothiophenes. The dibenzothiophenes are significantly more difficult to desulfurize in hydrotreating than diphenyl sulfides. However, DBT and presumably the substituted dibenzothiophenes are desulfurized readily by the bacterium Rhodococcus rhodocrous IGTS8. But when the dibenzothiophenes are highly diluted with other hydrocarbons in the diesel fuel, will there be enough specificity for their absorption and degradation by the enzyme systems? The extent to which a diesel fuel or middle distillate feed can be desulfurized by desulfurization enzymes was investigated. In addition, the effectiveness of molybdenum sulfide catalysts prepared at the Energy & Environmental Research Center for removing sulfur from diesel fuels was determined. A catalyst preparation involve creation of the molybdenum sulfide in a highly dispersed form between the layers of a mixed oxide (hydrotalcite) material. The mixed oxide can then be dissolved away to expose the molybdenum sulfide active sites.