Adsorption and reaction of 1-butene, cis- and trans-2-butene, and isobutene on silicalite H-ZSM-5 and H-FER were studied by FT-IR spectroscopy. On silicalite the different butenes were only physisorbed and no interaction took place. Over H-ZSM-5, hydrogen-bonded cis- and trans-2-butenes were initially formed followed by dimerization. Over H-FER the adsorbed 1-butene was converted into cis- and trans-a-butenes which were hydrogen-bonded to the acidic OH-groups of the zeolite and no further dimerization was found. Since the Bronsted acidity of H-FER and H-ZSM-8 is comparable it can be concluded that the pore size and pore structure and not the acidity plays the important role for the observed behavior. From this study and literature results, a monomolecular mechanism is proposed for the isomerization of n-butenes to isobutene over both types of zeolites (H-ZSM-5 and H-FER).