The oxidation reactivities of various alkyl dibenzothiophenes (DBTs) were evaluated and compared by measuring the rate constant of each alkyl DBT in oxidative desulfurization of a hydrotreated middle distillate (HMD) with cumene hydroperoxide at 75 degrees C over a MoO3/Al2O3 catalyst. It was found that the major sulfur compounds with higher abundance in HMD are the DBTs with alkyl substituents at the 4- and/or 6-positions, and the oxidative reaction of each alkyl DBT follows a pseudo-first-order reaction. The measured rate constants indicate that the oxidation reactivity of these alkyl DBTs decreases in the order of 4-MDBT > 2,4,6-TMDBT approximate to 4,6-TMDBT > 1,4,6-TMDBT approximate to 4,6-DMDBT > 4-E,6-MDBT. Comparison of the molecular structures of these sulfur species with their reactivities implies that the oxidation activity of alkyl DBTs with oil-soluble oxidant over a solid catalyst is mainly dependent on the steric hindrance around the central sulfur atom caused by the alkyl substituents at the 4- and/or 6-positions.