Wet embedding of crystals of zeolite beta in silica-alumina matrices produces effective catalysts for gas oil cracking which maintain catalytic activity after treatment under 100% steam at 1023 K. The same hydrothermal treatment applied to mechanical mixtures of zeolite and matrices and to zeolite beta embedded in pure silica matrices leads to loss of activity. The stabilizing effect of the silica-alumina matrix component has been investigated on model catalysts using XRD, nitrogen adsorption, FTIR, Al-27, and Si-29 MAS NMR, TG and acidity measurements. It is shown that the procedure of catalyst preparation results in extensive re-alumination and recrystallization of the zeolite framework, healing silanol defect groups and generating new structural Bronsted acid sites. This process involves migration of labile aluminum atoms from the matrix during the calcination of the zeolite/matrix gel mixture and occurs at a temperature below 600 K. A reaction mechanism, involving an equilibrium between the zeolite framework and hydroxylated aluminum species, is proposed.