The oxidation of model sulfur compounds (thiophene derivatives, benzothiophene derivatives, and dibenzothiophene derivatives), straight run-light gas oil (SR-LGO, S: 1.35 wt %), and vacuum gas oil (VGO, S: 2.17 wt %) were conducted with a mixture of hydrogen peroxide and formic acid. The thiophene derivatives with 5.696 to 5.716 electron densities on the sulfur atoms could not be oxidized at 50 degreesC. Benzo[b]thiophene with 5.739 electron density and other benzothiophene and dibenzothiophenes with higher electron densities could be oxidized. The sulfur compounds in SR-LGO and VGO appeared to be oxidized to a detectable levels (c.a., 0.01 wt % S) by GC-FPD analysis. The IR spectra of oxidized SR-LGO and VGO showed that sulfones were formed by oxidation. The removal of sulfur compounds by extraction became more effective for the oxidized samples than for the original samples. Lighter sulfur compounds were preferentially extracted. The extraction efficiencies of solvents, i.e., N,N'-dimethylformamide (DMF), acetonitrile (ACN), methanol, varied greatly. The most effective solvent for the removal of sulfur compounds was DMF. The recovery of oil was, however, lowest with DMF.