ZSM-22s, ion-exchanged with various cations, were prepared and examined as catalysts with respect to the skeletal isomerization of 1-butene to isobutene in a continuous flow reactor. Mg-ZSM-22 gave the best yield for isobutene. The order of the catalysts with respect to the yield for isobutene was proportional to the ratio of Lewis acidity to Bronsted acidity. The Lewis acid site was found to be responsible for improvement in catalytic activity, as well as suppression of coke formation. The role of Lewis acidity in the skeletal isomerization of 1-butene was confirmed by experiments involving the addition of water to the feed. The effect of impregnated boron or phosphorus was clearly observed. Phosphorus poisoned not only the strong acid sites, but also the active sites responsible for the skeletal isomerization of l-butene, while boron mainly poisoned the strong acid sites, which resulted in a decrease in side reactions and an increase in isobutylene selectivity.