Abstract: Zeolite Y comprising aluminum ions supplied and fixed by ion-exchange is produced by subjecting zeolite NaY to a series of treatment steps comprising e.g. ammonium ion-exchange and calcination. In order to supply and fix the aluminum ions, an aluminum ion-exchange is carried out, followed by further calcination in a saturated or substantially saturated steam (water vapor) atmosphere at 400.degree.-900.degree. C. If the zeolite unit cell size is to be reduced to 24.15-24.35 .ANG., simultaneously as the aluminum ions are fixed, the last-mentioned calcination is carried out in a saturated or essentially saturated (steam) water vapor atmosphere at 500.degree.-900.degree. C. It is further disclosed that the steam used in the calcination step can be supplied either from an external source or solely by vaporization of water present in the zeolite.