Preparation and structural characterization of a novel polyoxometalate (POM), [(P2W15Ti3O60.5)(4)(NH4)](35-) 1, i.e., an encapsulated NH4+ cation species in the central cavity of a tetramer (called the Dawson tetramer) constituted by trititanium(IV)-substituted alpha-Dawson POM substructure, are described. POM 1 was synthesized by several different methods and unequivocally characterized by complete elemental analysis, thermogravimetric and differential thermal analysis (TG/DTA), FTIR spectroscopy, solution (N-15{H-1}, P-31, W-183) NMR spectroscopy, and X-ray, crystallography. First, POM 1 was synthesized by a reaction of NH4Cl in aqueous solution with a precursor, which was derived by thermal treatment of a monomeric triperoxotitanium(IV)-substituted Dawson POM, [alpha-1,2,3-P2W15(TiO2)(3)O-56(OH)(3)](9-) 2, for 3 h in an electric furnace at 200 degrees C. The encapsulated NH4+ cation in 1 was confirmed by N-15{H-1} NMR measurement and X-ray crystallography. As another synthesis of 1, a direct exchange of the Cl- anion encapsulated in [{alpha-1,2,3-P2W15Ti3O57.5(OH)(3)}(4)Cl](25-) 3 With the NH4+ cation was attained by neutralizing an aqueous solution containing 3 with the addition of aqueous NH3 (the initial pH of ca. 2-2.5 was changed to 6.4), followed by adding NH4Cl. It has been clarified that the conditions as to whether the anion or the cation is encapsulated in the central cavity of the Dawson tetramer were significantly related to the protonation/deprotonation of the bridging oxygen atoms on the intramolecular surface, Ti-O-Ti/Ti-OH-Ti sites constituting the Dawson subunits.