This study presents a comparison of the catalytic performances and stabilities of H-Beta and H-USY zeolites for iC(4)/C-4 alkylation in supercritical isobutane phase. The experimental results clearly show that the zeolite structure strongly influences the performance and deactivation behavior of the zeolite in iC4/C4 alkylation performed in SC conditions. H-Beta outperformed H-USY zeolite as regards to its lower deactivation and the stable quality of the alkylate, composed mainly of isoparaffins while olefin di-/oligomerization was shown to be responsible of the H-USY deactivation with time-on-stream. Characterization of the spent catalysts gave clues to the differentiated performance of the two zeolites. The stable activity of H-Beta was attributed to the effective extractive effect of the supercritical iC(4) media to "clean" the acid sites located on the external surface of the small crystallites of H-Beta or located at the pore mouth. By contrast, the deactivation observed for H-USY was caused by olefin oligomerization inside the zeolite supercages. The supercritical isobutane media was shown to be unable to prevent the oligomers formation inside the zeolite supercages and/or to in situ extract these oligomers located in the supercages of the large crystals of the H-USY zeolite. (c) 2007 Elsevier B.V. All rights reserved.