The aluminum coordination in amorphous silica-alumina with different aluminum concentrations as well as in pure gamma-Al2O3 has been studied by means of Al-27 (MQ) MAS NMR spectroscopy. The effect of ammonia adsorption at 120 degreesC has been studied and compared with that on zeolite H-USY, as well as with zeolites with different topologies. As for protonic zeolites, part of the octahedral aluminum reversibly changed to tetrahedrally coordinated upon ammonia adsorption on amorphous silica-alumina. The distinction between flexible aluminum coordination in zeolites and silica-alumina was possibly based on the quadrupolar parameters. Ammonia adsorption on pure)gamma-Al2O3 did not influence the aluminum coordination. A parallel between the structure of zeolite USY and that of amorphous silica-alumina was made on the basis of the results of Al-27 MQ MAS NMR spectroscopy. A partial conversion from octahedral to tetrahedral coordination upon ammonia adsorption was observed for zeolite H-USY. Most of the flexible aluminum species belongs to an amorphous silica-alumina phase, formed during the ultrastabilization process from the reaction between the extracted silicon and aluminum.