The effect of hydrothermal treatment and mild acid leaching on the physico-chemical properties of zeolite Y and its vacuum gas oil hydrocracking performance was investigated. Ultra-stabilized Y (USY) zeolites were obtained by steam-calcination at 500, 600 and 700 °C. Steam-treated zeolites were further subjected to a mild acid leaching treatment. The zeolite samples were characterized by XRD, elemental analysis, XPS, N2 adsorption, 29Si and 27Al NMR and FTIR spectroscopy of adsorbed pyridine. Steam-calcination resulted in dealumination and with increasing severity the micropore surface area and the framework Al content decreased. At the same time, the Al content at the zeolite crystal surface increased. Acid leaching improved the pore accessibility and acid properties due to the extraction of extraframework Al species (EFAl). NiMoP-based hydrocracking catalysts were prepared from the modified USY zeolites with alumina as binder. Hydrocracking activities correlated with the acidity of the zeolites. Too severe steam treatment led to depopulation of acid sites and lowered the hydrocracking performance. Hydrocracking catalysts based on the acid leached zeolites were more active than the ones based on the corresponding steam-treated zeolites. It is based on the removal of agglomerated extraframework Al species that block the access to some of the micro- and mesopores. This study points out that, aside from the acidity, also other parameters such as pore accessibility and the presence of EFAl have considerable influence on the hydrocracking of the heavy molecules in a gas oil feed.