n-Butene was isomerized to isobutene over fresh and used ferrierite catalysts. Activity and selectivity for skeletal isomerization and biproduct formation were measured and changes following extended use were related to changes in ferrierite’s aluminum distribution. In its fresh form, ferrierite exhibited high activity for n-butene conversion and high selectivity for non-C-4 products with reduced selectivity for isobutene. With time on feed, side-product formation decreased and isobutene yields reached a maximum, then declined with further time on feed. After several run/regeneration cycles, a marked drop-off in the maximum achievable isobutene yield was observed and selectivity for non-C-4 products increased. MAS-NMR of the deactivated catalyst showed a decrease in zeolitic, tetrahedral aluminum and an increase in octahedral aluminum. In order to understand changes occurring following extended use, ferrierite powders were modified by steam and by steam plus acid-wash treatments. A steamed ferrierite catalyst exhibited lower initial n-butene conversion; however, selectivity to non-C, products increased relative to that of the fresh catalyst. XRD peak areas and gravimetric adsorption data indicated excellent crystallinity retention following steam treatment; XRD unit cell determination, and FTIR and NMR analysis suggested significant framework dealumination and the creation of a nonframework aluminum phase. TEM suggested that at least a portion of this phase exists in mesopores created by steam treatment. A steamed-acid-washed ferrierite catalyst also exhibited reduced initial n-butene conversion but high isobutene selectivity throughout the run. Biproduct formation was greatly reduced. Characterization indicated that acid-wash treatment after steaming had no effect on framework aluminum but partially reduced the amount of nonframework aluminum. Steamed-acid-washed ferrierite was much less sensitive to subsequent hydrothermal treatments than was the starting ferrierite. The study suggests a catalytic role for nonframework alumina, generated upon extended use of unmodified ferrierite and present in steamed ferrierite. Catalytically active, nonframework sites contribute to the production of non-C-4 products and reduced isobutene selectivity. Steamed-acid-washed ferrierite with the active nonframework phase removed exhibits excellent isobutene selectivity and catalytic stability.