A mechanistic model of surface reactions has been developed to describe the kinetic behavior and the stereochemical product distribution for the gas-phase hydrogenation of o-xylene over Pd/Al2O3. Models considering competitive, noncompetitive, and semicompetitive adsorption between hydrogen and o-xylene, featuring stepwise and pairwise addition of hydrogen atoms to the adsorbed o-xylene molecule, were applied. The stereochemical distribution of the products (cis- and trans-dimethylcyclohexane) was determined by the surface behavior of a cyclic olefin (reaction intermediate). This olefin is either directly hydrogenated to cis-DMCH or hydrogenated via a rollover mechanism to trans-DMCH. The stereochemical distribution was dependent on both temperature and the partial pressures of the reactants, which was explained by the advanced mathematical model. The model considering competitive adsorption turned out to better explain the stereochemical product distribution than the noncompetitive model. An extension of the competitive model, for example, the semicompetitive model, which allowed hydrogen to adsorb between adsorbed organic molecules, was also capable of explaining the main kinetic regularities, although with a slightly lower accuracy for the estimated parameters. (c) 2005 Elsevier Inc. All rights reserved.