Various dialkyldibenzothiophenes containing methyl, ethyl, diisobutyl, and diisopropyl substituents in the fourth and sixth positions were synthesized and their reactivities compared over an NiMo/Al2O3 industrial catalyst. Studies were made in a batch reactor at 573 K and at a total pressure of 5 MPa, Kinetic studies and competitive reactions demonstrated that adsorption on the surface of the catalyst was not the rate-determining step for their transformation. Hydrodesulfurization was proposed to proceed via a network of parallel reactions after partial hydrogenation of one aromatic ring of the sulfur compound. Variations in the reactivities of the different dibenzothiophene derivatives are well correlated to the steric hindrance generated by the alkyl groups near the sulfur atom. Moreover, 4,6-diisopropyldibenzothiophene showed very low reactivity, and the steric effect led to the disappearance of desulfurization under our reaction conditions.