Heteropolyacid hydrates of the general form H3PM12O40. nH2O are of interest to researchers in areas such as proton conductivity and catalysis. The water molecules in these hydrated acids reside in the interstitial volume between the large Keggin anions and can, under certain conditions, complex with the acidic protons to form stable cations such as the “dioxonium” ion (H5O2
+). We have used incoherent inelastic neutron scattering (IINS) measurements of dodecatungstophosphoric acid hexahydrate (H3PW12O40 · 6H2O) to obtain the vibrational density of states spectrum of H5O2
+ stabilized by the PW12O40
3− Keggin anions. Spectra were measure for fully hydrogenated and fully deuterated ions in order to determine the relative merit of previous assignments of the normal mode vibrations.