Mg-ZSM-22, dealuminated by steaming or washing with nitric acid, was prepared and investigated as a catalyst for the skeletal isomerization of l-butene to isobutene in a fixed bed flow reactor. The prepared catalysts were characterized by XRD, NH3-TPD, FT-IR, XPS, ICP, and FT-NMR. Steaming led to a significant decrease in acidity. Higher steaming temperatures removed more aluminum ions in tetrahedral sites and reduced the acidity. Both Bronsted acid sites and Lewis acid sites are reduced by steaming. The catalytic activities of steamed Mg-ZSM-22 were poorer than for untreated Mg-ZSM-22. Acid-washing with dilute nitric acid was effective for this reaction and showed enhanced activities. Strong acid sites, on which induced side reactions and coke deposition mainly occurred, were selectively removed and Lewis acidity, which is responsible for improvement in catalytic activity, was also generated by the treatment of Mg-ZSM-22 with dilute nitric acid.