To study the hydrodesulfurization (HDS) reaction network of 4,6-dimethyldibenzothiophene (DMDBT), three hydrogenated intermedia-testetrahydro-, hexahydro-, and dodecahydro-DMDBT-were synthesized, and their HDS was investigated over sulfided Mo and NiMo on gamma-Al2O3 catalysts at 300 degrees C and 5 MPa. Tetrahydro-DMDBT reacted by hydrogenation to hexahydro-DMDBT, which in turn reacted to dodecahydro-DMDBT by hydrogenation and to 3,3 '-dimethylcyclohexylbenzene by desulfurization. All four diastereoisomers of hexahydro-DMDBT were observed, all of which interconverted rapidly during HDS. Dodecahydro-DMDBT reacted by desulfurization to 3,3 '-dimethylbicyclohexyl. The rate constants of all steps in the kinetic network of the HDS of DMDBT could be measured over Mo/gamma-Al2O3, and those of some steps could be measured over NiMo/gamma-Al2O3. The first step-hydrogenation of DMDBT to tetrahydro-DMDBT-is the slowest, rate-determining step, but the hydrogenation of and the sulfur removal from hexahydro-DMDBT are also slow. Opening of the sulfur-containing ring in DMDBT and its hydrogenated intermediates occurs by C-S hydrogenolysis rather than by elimination. (c) 2007 Elsevier Inc. All rights reserved.