The dehydrogenation and cracking reactions of isobutane over zeolite HZMS-5 were studied at the DFT/B3LYP level of calculation. The zeolite was represented by the 'double-ring' 20T cluster. The activation energies for the reactions were 9-12 kcal/mol lower than those obtained with the linear 5T cluster. In both cases the attack of the acid site proton was directly on a carbon atom of the substrate, and not on the C-H and C-C bonds, evidencing carbonium-ion-type transition states. The results suggest that the reactions should be competitive, although the more hindered acid sites should favor the dehydrogenation over the cracking reaction. (C) 2003 Elsevier Science B.V. All rights reserved.