Three new organic–inorganic hybrid materials based on two important heteropolyoxometalates namely Preyssler (=K12.5H1.5[Na(H2O)P5W30O110]·35H2O) and Wells–Dawson (=K6[P2W18O62]·10H2O) anions, namely, (Hpro)9(Hleu)3K2[Na(H2O)P5W30O110]·25H2O (1), (Hpro)4(Hasp)[HP2W18O62]·20H2O (2), and (Hpro)11K3[Na(H2O)P5W30O110]·18H2O (3) where pro, leu, and asp are proline, leucine, and asparagine, respectively, were prepared and identified by elemental analysis, infrared and proton nuclear magnetic resonance spectroscopies, and thermogravimetric analysis. The hybrid materials are made up of positively charged amino acids, [Na(H2O)P5W30O110]14− and [P2W18O62]6− anions, and H2O molecules of crystallization. These constituents’ fragments held together into a three-dimensional supermolecular network through non-covalent interactions. The protonation constants of the amino acids used, and Preyssler and Wells–Dawson species in all possible protonated forms, the equilibrium constants for binary systems of proline–asparagine and proline–leucine, and the stoichiometry and stability constants of the corresponding binary and ternary hybrids with Preyssler and Wells–Dawson heteropolyoxometalates in aqueous solution were investigated by potentiometric pH titration method. The stoichiometries of the most hybrid species in solution were compared with the corresponding hybrids in the solid phase, in detail.

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