The isotopic exchange between hydrogen and deuterium was studied over a Pd/alumina catalyst. A model was proposed, including the adsorption and dissociation of hydrogen and deuterium. An adsorption-assisted desorption mechanism was also included and turned out to play a minor role in the H-2/D-2 exchange but have to be considered in the reaction between deuterium and water. A comparison between the experimental data and the simulation revealed that the model could describe the observed dissociation and isotope exchange reaction. (c) 2004 Elsevier B.V. All rights reserved.