In situ Al K edge XANES spectroscopy shows that the fraction of octahedrally coordinated aluminum in amorphous silica-alumina (ASA) and ultrastable Y zeolite (USY) decreases with increasing temperature under vacuum. In H-USY, about 10% of the aluminum remains octahedrally coordinated at 673 K, while, in ASA, virtually all the octahedrally coordinated aluminum is converted to tetrahedral coordination. In crystalline nonsteamed protonic zeolites, the fraction of octahedrally coordinated aluminum decreased to zero at 300 K. This is ascribed to the greater flexibility of the amorphous silica-alumina network in hosting water molecules and to the high concentration of silanol groups, which stabilize the hydrogen bonds. A large fraction of the nonframework aluminum in USY is amorphous silica-alumina.