Perpendicular adsorption of alkyl-substituted dibenzothiophenes on a Mo10S18 cluster was studied using density functional theory. The Mayer bond order between the sulfur atom of dibenzothiophene and the molybdenum atom of the Mo10S18 cluster was calculated from the optimized adsorption complex. Depending on the position of the alkyl substitution in dibenzothiophene, the values of the Mayer bond order were classified into three groups, and compared with the experimental hydrodesulfurization (HDS) rate constants measured at 320 degreesC, 70 atm, and 1.6 h(-1) over a NiMo/Al2O3 catalyst using a light cycle oil.