An analysis of the "mechanism" and the assumptions incorporated into the derivation of the Mars-van Krevelen rate expression for heterogeneously catalyzed oxidation reactions reveals a number of inconsistencies. The expression as derived is properly applicable only for a reaction involving molecular oxygen adsorbed on a single site; however, the model assumes that lattice 0 ions (or atoms) react with the substrate being oxidized. Furthermore, the steps describing the model are not elementary steps and, in addition, the competitive adsorption of intermediates and products is not considered. Consequently, the original derivation is incorrect and this rate expression must be viewed only as a mathematical data-fitting function. Alternative Langmuir-Hinshel wood, Hougen-Watson-type catalytic sequences can typically provide similar, and sometimes identical, rate equations which can fit rate data better than, or at least as well as, the Mars-van Krevelen expression. These latter models also provide rate parameters that can be evaluated for thermodynamic consistency. (c) 2007 Elsevier B.V. All rights reserved.