Oxidation of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied using the isobutyraldehyde/molecular oxygen oxidation system in the absence of metal catalyst as the sulfur present in hydrodesulfurized (HDS) diesel is mostly in the form of substituted dibenzothiophenes. This oxidation system was found to oxidize these sulfur compounds to their respective sulfones, and oxidation reactivity was found to decrease in the order 4,6-DMDBT > 4-MDBT > DBT. Further, the study was extended to oxidation of sulfur compounds present in HDS diesel in a two-phase system using the aldehyde/molecular oxygen oxidation system. The oxidized HDS diesel on extraction with polar solvents showed remarkable reduction in total sulfur. Acetonitrile was found to be the most effective among solvents studied, and isobutyraldehyde was observed to be the best. among aldehydes studied. In HDS diesel, total sulfur could be reduced from 448 to 77 ppm by oxidation followed by solvent extraction, and it could be further reduced to 31 ppm by passing through a silica gel column.