Adsorption and reaction of 2-methylpropene (isobutene) on zeolites (ZSM-5 and faujasite (Y zeolite)), a silica-alumina, and a silica were investigated below room temperature by infrared (IR) spectroscopy. On silica, isobutene molecules that adsorbed on silanol groups simply desorbed below 230 K in vacuum, while isobutene pi-bonded to the acidic OH groups (pi-complex) on acid catalysts reacted to dimer alkoxy (2,4,4-trimethylpentoxy) groups. The structure of the reaction product, dimer alkoxy groups, was found to be restricted by the pore size of zeolites; 2,4,4-trimethyl-2-pentoxy species were identified on silica-alumina and Y zeolite similarly to those on mordenite, while the reaction of isobutene resulted in 2,4,4-trimethyl-1-pentoxy species on the acidic OH groups in the pores of ZSM-5. Therefore, the space restriction of the pore size of zeolites on the reaction product was directly observed by the low-temperature IR study.