Oxidative desulfurization of diesel fuels with molecular oxygen was examined by using cobalt salts and aldehydes as catalysts and sacrificial materials, respectively. At first, the authors conducted desulfurization of model oils consisting of benzene and dibenzothiophene. A mixture of benzene, dibenzothiophene, n-octanal, and an appropriate cobalt salt (acetate or chloride) was stirred at 40 degreesC under atmospheric pressure of oxygen to afford dibenzothiophene sulfone in almost quantitative yield within 15 min. Dibenzothiophene sulfone produced could be easily removed from the model oils by silica or alumina adsorption. Several organic sulfides including thioanisole, diphenyl sulfide, benzothiophene, and 4,6-dimethyldibenzothiophene also could be converted to the corresponding sulfones in almost quantitative yields. Then, the authors examined ultra deep desulfurization of a commercial diesel fuel, which contains 193 wt ppm of sulfur. By using the system consisting of cobalt acetate, aldehyde, and molecular oxygen, sulfur-containing compounds in the diesel fuel were oxidized, and then removed by alumina adsorption and/or solvent extraction. The resulting oil contained less than 5 wt ppm of sulfur; this corresponds to the result that more than 97% of sulfur in the oil could be removed. These results may indicate that this brand-new oxidative desulfurization process has a potential to meet a future regulation of sulfur in the diesel fuel.