The mechanism of zeolite Y destruction by steam in the presence of vanadium is described. Electron spin resonance, UV-VIS diffuse reflectance, and sorption measurements are used to understand vanadium dynamics on the zeolite. Vanadium deposited on the external surface of the zeolite migrates into the channels by being heated in oxidizing atmosphere; although water helps vanadium reach the acid sites, it is not required. Vanadium is stabilized as VO2+ cation near the acid sites. The strongest acid sites can stabilize V as VO2+ cations, but experimental results show that V-IV does not play any role in zeolite destruction. Extraframework aluminum competes with the zeolite for vanadium and delays its migration to the acid sites. In the presence of water vanadic acid is formed inside the zeolite according to the reaction VO2+- Y + 2 H2O reversible arrow H+-Y + H-3 VO4. Since vanadic acid is a strong acid, it can destroy the zeolite by hydrolysis of the SiO2/Al2O3 framework; in this way, vanadium can act as a catalyst for zeolite destruction. Synergistic action between sodium and vanadium is explained. A detailed mechanism for zeolite dealumination by steam is proposed. (C) 1997 Academic Press.