The syntheses and molecular structures of the two types of alpha-Dawson-type trialuminum-substituted polyoxometalates, [B-alpha-H3P2W15O59{Al(OH2)}(3)](6-) (1) and [B-alpha-H3P2W15O59{Al(OH)}(2){Al(OH2)}](2)(16-) (2), are described herein. The potassium and cesium salts of 1, K-6[B-alpha-H3P2W15O59{Al(OH2)}(3)]center dot 14H(2)O (K-1), and Cs-6[B-alpha-H3P2W15O59{Al(OH2)}(3)]center dot 13H(2)O (Cs-1) were formed by a stoichiometric reaction in water of trilacunary alpha-Dawson polyoxotungstate with aluminum nitrate under acidic conditions (pH similar to 3). The potassium/sodium and tetramethylammonium/sodium salts of 2, K14Na2[B-alpha-H3P2W15O59{Al(OH)}(2) {Al (OH2)}](2) center dot 30H(2)O (KNa-2) and [(CH3)(4)N](14)Na-2[B-alpha-H3P2W15O59{Al(OH)}(2){Al(OH2)}](2) center dot 39H(2)O (TMANa-2) were obtained under basic conditions (pH similar to 9). These compounds were characterized by X-ray structure analyses elemental analyses, thermogravimetric/differential thermal analyses, Fourier transform infrared, and solution P-31, Al-27, and W-183 NMR spectroscopy. The polyoxoanion 1 is a monomeric, alpha-Dawson-type structure, resulting in an overall C-3v symmetry, while the polyoxoanion 2 is a hydrogen-bonded dimeric structure, resulting in an overall S-3 symmetry in the solid state. The pH dependence of polyoxoanions 1 and 2 in aqueous solution was also investigated by P-31 NMR spectroscopy.