Adsorption and reactions of 2-methylpropene (isobutene) on mordenite were investigated between 150 and 300 K by infrared (IR) spectroscopy. The methyl groups of isobutene interacted with the acidic OH groups of mordenite below 160 K, which was converted to the strongly interacted pi-bonded isobutene to the acidic OH groups (pi-complex). Above 200 K, the pi-bonded isobutene dimerized even under evacuation condition. The stable 2,4,4-trimethyl-2-pentoxy species was identified by FT-IR by comparison with the spectra of the adsorbed Cs olefins. The expected monomeric reaction intermediate, the 2-methyl-2-propoxy (tert-butoxy) species, was not observed. This is probably due to the rapid migration and reaction of the adsorbed isobutene molecules.