The skeletal isomerization of 1-butene has been studied on two MCM-22 zeolite catalysts synthesized with different Si/Al ratios, i.e., 15 and 47, under a wide range of operating conditions. Product yields are a function of n-butene conversion, and a maximum of isobutene yield is found at ca. 50% conversion. Besides isobutene, propylene and pentenes, which are formed by dimerization-cracking reactions, are the main products formed on MCM-22, while C-9 and heavier hydrocarbons are not observed in the reaction products. Isobutene also takes part, together with the n-butenes, in the formation of by-products through dimerization and/or codimerization processes, which would also lead to a lower isobutene selectivity. Moreover, the yield of isobutene increases with increasing reaction temperature and decreasing 1-butene partial pressure, while the contrary holds for the formation of by-products. On the basis of these results the mechanism of formation of isobutene and the different by-products on MCM-22 is discussed. Finally, the selectivity to isobutene is strongly improved by increasing the Si/Al ratio of the zeolite, which is explained by a decrease of the undesired consecutive reactions when decreasing the density of Bronsted acid sites of the zeolite.