A large amount of research in fluid catalytic cracking (FCC) has focused on understanding the destructive role of vanadium, since it is a major issue for catalyst performance during the cracking of residuum-containing feeds. Here, we establish a mechanism for the destruction of Y zeolite which ties the roles of sodium and vanadium together and explains their synergy. Understanding pathways for Y destruction is necessary to improve FCC performance. Y zeolite destruction in the presence of steam occurs via two pathways: steam hydrolysis of framework Al and direct attack by sodium species. For low Na+ Y zeolite, steam hydrolysis of framework Al is the main cause of zeolite collapse. In such a pathway, rapid healing of framework tetrahedral holes by Si species from silica-enriched matrices stabilizes the zeolite. In the second pathway, sodium is responsible for zeolite hydrothermal instability. Sodium reacts with steam to form an active center, a surface NaOH, for zeolite destruction. All our observations are rationalized by proposing that the key to destruction lies in the ease of formation and availability of NaOH. It is the basic OH- entity that attacks the framework Si-O bonds. Vanadium does not engender a new destructive pathway. At 970-1100 K in the presence of air and steam, V will be, as suggested by others, in a surface mobile state in an acidic form. This V species reacts with cationic sodium, facilitating its release from the Y exchange site. The sodium metavanadate thus formed hydrolyzes in steam to form NaOH and metavanadic acid, which may again react with Na+ cations. Here, as in the V-free case, NaOH is the destructive agent, with formation of the basic OH- entity again being the key. The role of vanadium is to catalyze and facilitate the formation of NaOH. And the mechanism of Y destruction, NaOH attacking Si-O, remains the same whether V is present or not. Without sodium, V itself has little effect on Y zeolite stability, regardless of zeolite unit cell size.