H-ferrierite was treated with oxalic acid to selectively remove external acid sites. The treatment led to the reduction of tetrahedrally coordinated aluminum ions and the increase of octahedrally coordinated aluminum species and micropore surface area. It was also found that the catalyst treated with oxalic acid is resistant to coking. Over modified H-FER, very high selectivity to isobutene is obtained at lower conversions suggesting the skeletal isomerization of I-butene is occurring through a mono-molecular reaction mechanism.