Zeolites La,Na-Y are important catalysts for industrial FCC units due to their acidic properties and stability. The ethylbenzene disproportionation on this zeolite was suggested as a standard test reaction for acidic zeolites by the Catalysis Commission of the International Zeolite Association (IZA). The present work provides solid-state (13)C NMR spectroscopic evidence that various amounts of reactants inside the supercages of zeolites La, Na-Y obviously affect the catalytic behavior of ethylbenzene reaction. At 453 K, diphenylethane was only detected on zeolite La, Na-Y with three ethylbenzene molecules per supercage. This intermediate, however, did not occur on samples with low loading (1 or 2 EB molecules per supercage). On zeolite La, Na-Y loaded with one molecule ethylbenzene per supercage, dealkylation and realkylation of ethylbenzene occurred at 548K without any side-reactions. However, side-reactions of oligomerization, hydride transfer, and aromatization were observed on samples with high ethylbenzene loading, which promotes the catalyst deactivation. In addition, the zeolites La, Na-Y with high loadings of ethylbenzene show a higher reactivity and more complex reaction mechanisms. (C) 2010 Elsevier B. V. All rights reserved.