The catalytic properties of the proton form of six different 10-ring zeolites (clinoptilolite, ferrierite, ZSM-22, SUZ-4, ZSM-57, and ZSM-5), together with the dealuminated analogs of some of these materials prepared via oxalic acid treatment, are compared in the skeletal isomerization of 1-butene. While the pore shape of 10-ring channels in this series of medium-pore zeolites was found to be the key parameter substantially governing the isomerization activity, the catalytic data obtained from two H-ferrierites with similar Si/Al ratios but different crystal sizes reveal that the rate of coke formation on this particular zeolite structure, as well as its isobutene selectivity, can differ significantly according to the zeolite crystal size. The overall results of our study strongly suggest that the remarkable isobutene selectivity of aged H-ferrierite is a consequence of pore mouth shape catalysis over the Brcircle dividensted acid sites located near the 10-ring pore mouths with a suitable degree of ellipticity, which may come not only from the unique geometrical constraints imposed by the dual pore system of this particular zeolite, but also from its behavior of being normally synthesized with a submicrometer crystal size. (C) 2004 Elsevier Inc. All rights reserved.