Zeolites show interesting properties as catalysts for MTBE synthesis from methanol and isobutene such as a high selectivity to MTBE even at a low methanol/isobutene feed ratio. In order to explain this high selectivity, the adsorption behaviors of HY and HZSM-5 zeolites and their impact on activity and selectivity for MTBE synthesis were studied. Adsorption experiments, carried out under conditions similar to those used for reaction, showed that ca. 2.5 molecules of methanol were adsorbed per acid site on HZSM-5 and HY zeolites, whereas isobutene was found to form a 1:1 adsorption complex. The excess methanol adsorbed was found to be only weakly bonded, probably via hydrogen bonds. On a commercially used resin catalyst (Amberlyst-15) equal amounts of methanol and isobutene were adsorbed. The higher methanol uptake of the zeolites was paralleled by a higher selectivity to MTBE as compared to the resin catalyst. The increased adsorption of methanol on the zeolites was concluded to play a key role in suppressing the formation of by-products due to isobutene dimerization or oligomerization by decreasing the adsorption of isobutene on the active sites and thereby keeping these sites available for reaction.