The synthesis of MCM-41 type materials with high amounts of tetrahedral aluminum is reported. Transmission electron microscopy with EDX elemental analysis shows that when using a tetraalkylammonium base in the synthesis of MCM-41 materials Si/Al ratios as low as 12 are possible. In all cases the MCM-41 materials contain an aluminum-rich dense phase in which the aluminum is also tetrahedrally coordinated. This dense phase consumes most of the aluminum of the synthesis mixture.