In order to develop a better understanding of methyl tert-butyl ether (MTBE) synthesis on zeolites, the coadsorption of methanol and isobutene on HY zeolite was investigated using IR spectroscopy. Initial adsorption of isobutene alone at 35 degrees C led to rapid oligomerization yielding strongly bound oligomers. The subsequent coadsorption of methanol did not induce any changes in the zeolite-adsorbate complexes. TPD following the coadsorption showed that the Broinsted acid sites could be restored by temperature treatment above approximately 300 degrees C. When methanol was adsorbed first and isobutene was subsequently coadsorbed, MTBE was formed even at 35 degrees C on the catalyst surface. MTBE desorbed easily at a temperature of 70 degrees C, restoring a major fraction of the Brosted acid sites. Methanol was concluded to decrease the probability of oligomerization by effectively competing for the acid sites.