An intense and narrow infrared band was observed around 885 cm(-1) in the spectrum of activated dealuminated beta zeolites. This band does not correspond to the vibration of a hydroxy group, as was shown by in situ deuterium exchange and heavy-water steaming of the sample in the infrared cell. It is rather linked to the vibration of a site corresponding to specific defects in the structure. Pyridine adsorption evidenced the strong Lewis character of this site. At the same time, a diner link with a nu(OH) band at 3782 cm(-1) was found. CO2 adsorption partially transformed these OH groups into hydrogen-carbonate species, thus showing their basic properties. Assignment is proposed to a tricoordinated aluminum atom partially connected to the framework, with a hydroxy group on the aluminum atom. The electronic vacancy on the Al atom, together with the basic hydroxy group, constitutes an acid-base pair on an acidic zeolite. The 885 cm(-1) band is a direct infrared fingerprint of a Lewis site.