Infrared spectra of dealuminated high silica MFI zeolites were measured to elucidate the structure of silanol groups active for conversion of 1-butene to propene. The OH groups were replaced by OD groups to have clear vibrational spectra. Two sharp bands and one broad band were observed; isolated silanol at 2746 cm(-1), terminal silanol at 2725 cm(-1), and hydrogen-bonded silanols centered at 2570 cm(-1) composed of more than one components. Based on the response of these bands to Na poisoning and pyridine adsorption together with the correlation with catalytic results, we propose that the active sites for propene formation are one of the terminal silanol groups and one of the hydrogen-bonded silanol groups in a silanol nest. The active hydrogen-bonded silanol groups are the one whose O atom is hydrogen-bonded to adjacent two H atoms in the silanol nest. Graphic