Al-27 MAS (magic angle spinning) and 3Q (triple quantum) MAS NMR (nuclear magnetic resonance) at a very high magnetic field (16 T, 700 MHz for H-1) enabled us to detect all the Al species on a series of modified Y zeolites. In a USY (ultra stable Y) zeolite after steaming, extra-framework Al species with various environments were detected, while only framework Al (IV) was observed in the parent NaY zeolite. 3QMAS NMR clearly identified the tetra-coordinated extra-framework Al with a large structural anisotropy (IVb), in addition to the penta-(V) and hexa-coordinated (VI) species. Na2H2-EDTA (ethylenediaminetetraacetic acid) treatment of the USY was found to convert the V and VI species into the IVb species. Increase in the Bronsted acid strength and the catalytic activity for octane cracking has been found on the Na2H2-EDTA-treated USY. Therefore, it is speculated that the IVb species is the origin of enhanced Bronsted acidity (and thereby the high cracking activity) of the USY zeolite. (c) 2005 Elsevier B.V. All rights reserved.