The effect of acid treatment on the catalytic performance of ultrastable Y catalysts in the Beckmann rearrangement of cyclohexanone oxime has been investigated under liquid phase conditions. Over the catalysts treated under mild conditions, enhanced activity was observed due to an increase in the amount of Bronsted acid sites, but a considerable amount of extra-framework aluminum (EFAL) with Lewis acidity remained, resulting in a rapid loss of selectivity to epsilon-caprolactam. Under relatively severe conditions, the extensive removal of EFAL improved the lactam selectivity, although the extraction of framework aluminum resulted in decreased oxime conversion. Pyridine and benzonitrile adsorption studies indicated the presence of at least two different kinds of Lewis acid sites in the catalysts. The relationships between the catalytic. performance and Fourier transform infrared spectroscopy suggested that weak Bronsted acid sites are the active centers for the rearrangement reaction, whereas weak Lewis acid sites due to an extra-framework amorphous phase are responsible for the cyclohexanone formation. Thus the catalyst with weak Bronsted acid sites and a small amount of acidic EFAL species is active and selective for the liquid phase Beckmann rearrangement of cyclohexanone oxime. (c) 2005 Published by Elsevier Inc.