The effects of Si/Al ratio of starting NaY zeolites on structural changes during ion exchange treatments were studied. Three zeolites of different Si/Al ratio were subjected to three cycles of ion exchange treatment using (NH 4 ) 2 SO 4 and subsequent calcinations, and analyzed after each cycle. In the first ion exchange, the structural changes of the zeolites were almost independent of the Si/Al ratio of the starting NaY zeolites. For all three zeolites, about 60% of Na ions were exchanged, and the relative crystallinity decreased to about 80% of the starting NaY. In the second and third ion exchanges, the structural changes depended on the Si/Al ratio of the starting NaY zeolites. The zeolitic framework of Y zeolite with a low Si/Al ratio of 2.4 deteriorated, whereas Y zeolite with a medium Si/Al ratio of 2.8 and Y zeolite with a high Si/Al ratio of 4.1 retained their high crystallinity even after the third ion exchange. Furthermore, among the three zeolites, Y zeolite with a medium Si/Al ratio of 2.8 showed largest degrees of ion exchange, dealumination, and mesopore formation.