In order to understand the structure of aluminum phosphates, prepared by the sol-gel method, 27Al and 31P MAS NMR spectroscopy has been employed. The evolution of structure during the gel-glass-crystal transformation has been followed as a function of both temperature and compostion. Aluminum is octahedrally coordinated in the xerogel and on heating it is partially converted into tetrahedrally coordinated Al in the gel glass. Phosphorous is present in metaphosphate units. The structure of crystalline AlPO4 is shown to consist of an ordered covalent network involving [PO
4
2
]+
and [AlO
4
2
]−
units.
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