Oligomerization of isobutene to produce oligomers such as trimers and tetramers has been investigated over dealuminated Y zeolite catalysts that were obtained by calcination at temperatures in the range of 500-700 degrees C in the presence of steam. Relatively stable isobutene conversion and high selectivity for trimers and tetramers are attained over a dealuminated Y zeolite by steam-treatment at 600 degrees C. The observed stable performance with the increased selectivity is attributed to the high concentration of Lewis acid sites. Moreover, a deactivated catalyst can be regenerated easily by simple calcination in air. The present study demonstrates that a steamed zeolite having high concentration of Lewis acid sites, even with decreased crystallinity and porosity, is one of the potential catalysts to exhibit stable and high conversion, high selectivity (for trimers and tetramers) and facile regeneration for the oligomerization of isobutene. However, the contribution of a strong or new acid site for the stable catalysis cannot be ruled out. (c) 2007 Elsevier B.V. All rights reserved.