Isotopically labeled reactants (D-2 and C3D8) were used in order to probe H2S formation pathways during thiophene desulfurization on Co/H-ZSM5 at 773 K using H-2 or propane as co-reactants. With D-2/C4H4S or C3D8/C4H4S reactants, both D and H were present in the hydrogen sulfide formed, suggesting that desulfurization can occur via both direct thiophene decomposition with intramolecular hydrogen transfer and deuterium addition from D-2 or C3D8. Thiophene, however, becomes readily deuterated via rapid exchange with D-containing intermediates formed from D-2 or C3D8, suggesting that thiophene protonation-deprotonation steps are fast. These isotopic equilibration processes prevent a definitive assessment of the contributions of these two pathways to hydrogen sulfide formation. The D-content in hydrogen sulfide and in hydrocarbons are higher with C3D8/C4H4S than with D-2/C4H4S reactants, indicating that propane is more effective than dihydrogen as a source of hydrogen for desulfurization reaction. C3H8/C4H4S and C3D8/C4H4S reactants gave similar reactions rates for hydrocarbon formation, suggesting that C-H bond activation steps do not limit hydrocarbon formation rates. A normal kinetic isotope effect was observed for thiophene desulfurization, indicating that the abstraction of fragments from propane-derived intermediates for reactions with thiophene is more difficult for deuterated intermediates. A comparison of thiophene desulfurization rates with C3H8/D-2/C4H4S, C3H8/C4H4S, and D-2/C4H4S mixtures confirmed that propane is a more effective source of hydrogen than H-2. The deuterium content in all products formed from C3H8/D-2/C4H4S mixtures was lower than in products formed from C3H8/D-2 reactants. These findings suggest that thiophene inhibits the hydrogen adsorption-desorption steps responsible for the incorporation of D-atoms into adsorbed intermediates from D-2 and for the desorption of dihydrogen during propane reactions.