The Langmuir-Hinshelwood (LH) rate expression is often used to describe the kinetics of heterogeneously catalyzed reactions using zeolites. A factor theta v &3bond; [1 - Sigma(i)theta(i)] in the LH expression allows for the reduction of the reaction rate with increased fractional occupancies theta(i) of the individual species on the surface. Most commonly in practice the multi-component Langmuir (MCL) approach is used for calculation of the fractional occupancies giving theta(v) &3bond; 1/[1 + Sigma(i) b(i)f(i)] where the b(i) are the Langmuir adsorption constants and f(i) are the component fugacities in the gas phase. The LH-MCL approach is however thermodynamically consistent only when the saturation capacities of all the individual species in the mixture are identical to one another, or when the component loadings are small. For mixtures containing molecules with different saturation capacities, the sorption loadings are significantly affected by entropy effects, especially for high loadings within the zeolite catalyst. In the general case, we need to determine the sorption loadings, and occupancies, using the Ideal Adsorbed Solution Theory (IAST). Using the gas phase isomerization of n-hexane with MFI zeolite catalyst as an illustration we demonstrate the limitations of the LH-MCL kinetics for calculation of the catalyst effectiveness factor. The differences between the classical LH-MCL and LH-IAST approaches increase at high loadings inside the catalyst pellet. The important consequences for design of fixed bed reactors are also illustrated. (c) 2005 Elsevier B.V. All rights reserved.