Just as in the parent Na+-MCM-41, all the aluminum in NH4+-MCM-41 with structural Si/Al = 10-60 is in 4-coordination. Upon calcination in air, H+-MCM-41 is formed and a small part of aluminum is removed from the aluminosilicate structure. We conclude that structural aluminum in MCM-41 is thermally less stable than, for example, in zeolite Y, because the MCM-41 structure lacks strict crystallographic order at the atomic level and the very small H+ cations cannot satisfy the framework charge balance as efficiently as the Na+ cations.