Al-MSU-S-FAU mesoporous molecular sieve catalysts with Al contents ranging from 2.5 to 50 mol % and different pore sizes have been prepared from solutions of nanoparticular zeolite "seeds" and C-12, C-14, and C-16 templates. Significantly higher amounts of tetrahedrally coordinated Al are incorporated into the Al-MSU-S-FAU structure over typical Al-MCM-41-type materials. At high Al loadings, Al-MSU-S-FAU materials exhibit better hexagonal pore ordering than Al-MCM-41 materials, but pore diameters are observed to decrease with increasing Al content. Al-MSU-S-FAU materials contain significant volumes of irregular mesovoids randomly distributed within the primary particles, the volumes of which increase with increasing Al content. Catalytic cumene cracking activity is very high over the low Al content Al-MSU-S-FAU materials but decreases in direct correlation with the increasing Al content. Hence, reduced accessibility to the Al centers, or decreasing catalytic activity of the Al centers as the Al content increases, is suggested as a result of the formation of small-pore, high-Al zeolite seeds of zeolite A or sodalite, where Al centers are not accessible to larger cumene reagents.