The reaction molecularity of the skeletal isomerization on zeolite was deduced from IR spectra of adsorbed butene recorded with heating. Formation of polymeric species from adsorbed butene was observed on ZSM-5 and beta zeolites, non-selective catalysts for isobutene production. On the other hand, most of butene molecules adsorbed on ferrierite and clinoptilolite behaved monomolecularly, resulting in complete desorption even at ambient temperature. The exceptional selectivity for isobutene of ferrierite and clinoptilolite was ascribed to the sparse distribution of adsorbed butene molecules, suppressing bimolecular oligomerization and enhancing monomolecular skeletal isomerization.