The reaction mechanisms of the H-exchange and methylation of benzene catalyzed by various Lewis acid catalysts are investigated through DFT calculations. The reactions are concerted and no stable charged intermediates are formed. Reaction rate constants and HSAB-based reactivity descriptors are calculated to compare the activity of the different Lewis acids. AlCl3 is always more active than BF3, in agreement with experimental results. The ranking of Al(OH)(3) and the L-1 site of zeolites, which have comparable active centers, in this activity sequence is less obvious. Sigma(Deltas(AB))(2) values appear to be a good reactivity descriptor for describing the intermolecular reactivity sequences of these reactions. (C) 2003 Elsevier Inc. All rights reserved.