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Combined high resolution electron microscope (HREM) and analytical electron microscope (AEM) investigations have been used to study the formation and evolution of mesopores in hydrothermally dealuminated ultrastable Y (USY) materials. The HREM results give clear evidencc for an inhomogeneous formation and distribution of the 5- to 50- nm defect regions attributed to mesopores. Such features are characteristic of extended hydrothermal treatment. In regions with high defect concentration, mesopores coalesce to form channels and cracks, which ultimately define the boundaries of fractured crystallite fragments. At these boundaries, a dark band is often observed which is highly enriched in aluminum, while within the mesopore itself, aluminum appears to be deficient. These dark bands are observed both in a neat USY material subjected to several cycles of steam acid treatment and in a high-temperature steam-deactivated USY fluid catalytic cracking (FCC) catalyst. Individual grains of both the neat USY and the dealuminated USY catalyst exhibit extreme fracturing into small crystallites of dimensions 20 to 50 nm. The fracturing is attributed to the nonequilibrium nature of the high-temperature steam treatments which characterizes accelerated deactivation procedures in the laboratory.