Tungsten oxide species form strong acid sites on ZrO2 supports and inhibit ZrO2 crystallite sintering and tetragonal to monoclinic structural transformations. W-L-I X-ray absorption near-edge spectra suggest that the W centers are in a distorted octahedral oxygen environment, even after dehydration at 673 K, in all WOx-ZrO2 samples(2-21 wt.% W) oxidized at 1073 K. Maximum o-xylene isomerization turnover rates (per W atom) on WOx-ZrO2 solids occur at WOx surface densities (10 W nm(-2)) that exceed the theoretical monolayer capacity of ZrO2. Similar turnover rates are obtained on WOx-ZrO2 samples with similar WOx surface densities (W nm(-2)) over a large range of oxidation temperatures (773-1223 K) and WOx concentrations (5-21 wt.% W). UV-visible spectra suggest an increase in WOx domain size with increasing surface density. High isomerization turnover rates appear to require the presence of WOx domains of intermediate size on ZrO2 surfaces. WOx domains of intermediate size appear to provide a compromise between reducibility and accessibility of WOx centers. These domains are necessary to delocalize a temporary charge imbalance that forms Bronsted acid sites in the presence of H-2 and stabilizes carbocation intermediates. The presence of H-2 during o-xylene isomerization increases turnover rates and prevents rapid deactivation. Slow D-2/o-xylene exchange reactions indicate that H atoms from H-2 are not frequently involved in the activation or desorption of xylenes. H-2 is required, however, in order to reverse the occasional desorption of H atoms during o-xylene isomerization reactions. These desorption processes lead to the destruction of Bronsted acid sites by the formation of strongly adsorbed unsaturated species in the absence of H-2. After promotion with Pt (0.3 wt.%), WOx-ZrO2 solids catalyze n-heptane isomerization in the presence of H-2 at 400-500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. On Pt/WOx-ZrO2, efficient hydrogen transfer steps prevent extensive cracking of adsorbed carbocations by limiting their surface lifetimes.