The effect of H-2 on isomerization pathways and on the acid site density on WOx-ZrO2 catalysts was explored using kinetic measurements of acid-catalyzed o-xylene isomerization reactions. Initial o-xylene isomerization rates on WOx-ZrO2 at 523 K are proportional to the H-2 concentration even though H atoms are not involved directly in reaction turnovers, as shown by the low deuterium content in the isomers formed from D-2/o-C8H10 mixtures. H-2 chemisorption uptakes on WOx-ZrO2 and isomerization rates show a similar dependence on WOx surface density and structure, suggesting that Bronsted acid sites form via reductive processes that require H atoms from H-2. This proposal is consistent with UV-visible spectra, which show the formation of reduced centers by H-2; these centers correspond to the formation of acidic Hdelta+ species with charge compensation by WO3delta- domains. The transients induced by the addition and removal of H-2 (or O-2) during xylene isomerization show that the formation of acid sites by H-2 is reversible at reaction conditions and that H-2 is involved in maintaining a steady-state density of acid sites. H-2 also inhibits catalyst deactivation, apparently by reversing C-H bond activation steps leading to the formation of strongly adsorbed unsaturated hydrocarbons. A sequence of elementary steps including isomerization, deactivation, and acid site formation pathways was found to describe accurately the observed effects of H-2 and o-xylene on deactivation, reaction rates, and the results of o-(C8H10)-C-13/o-C8H10 and D-2/o-C8H10 exchange experiments.