Aluminium dihydrogenphosphate describes inorganic compounds with the formula Al(H<sub>2</sub>PO<sub>4</sub>)<sub>3</sub><sup>.</sup>xH<sub>2</sub>O where x = 0 or 3. They are white solids. Upon heating these materials convert sequentially to a family of related polyphosphate salts including aluminium triphosphate (AlH<sub>2</sub>P<sub>3</sub>O<sub>10</sub><sup>.</sup>2H<sub>2</sub>O), aluminium hexametaphosphate (Al<sub>2</sub>P<sub>6</sub>O<sub>18</sub>), and aluminium tetrametaphosphate (Al<sub>4</sub>(P<sub>4</sub>O<sub>12</sub>)<sub>3</sub>). Some of these materials are used for fireproofing and as ingredients in specialized glasses.
According to analysis by X-ray crystallography, the structure consists of a coordination polymer featuring octahedral Al<sup>3+</sup> centers bridged by tetrahedral dihydrogen phosphate ligands. The dihydrogen phosphate ligands are bound to Al<sup>3+</sup> as monodentate ligands.