In this paper we report the first example of peptide hydrolysis catalyzed by a polyoxometalate complex. A series of metal-substituted Wells-Dawson polyoxometalates were synthesized, and their hydrolytic activity toward the peptide bond in glycylglycine (GG) was examined. Among these, the Zr(IV)- and Hf(IV)-substituted ones were the most reactive. Detailed kinetic studies were performed with the Zr(IV)-substituted Wells-Dawson type polyoxometalate K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which was shown to act as a catalyst for the hydrolysis of the peptide bond in GG. The speciation of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which is highly dependent on the pD, concentration, and temperature of the solution, was fully determined with the help of (31)P NMR spectroscopy and its influence on the GG hydrolysis rate was examined. The highest reaction rate (k(obs) = 9.2 (±0.2) × 10(-5) min(-1)) was observed at pD 5.0 and 60 °C. A 10-fold excess of GG was hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O proving the principles of catalysis. (13)C NMR data suggested the coordination of GG to the Zr(IV) center in K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O via its N-terminal amine group and amide carbonyl oxygen. These findings were confirmed by the inactivity of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O toward the N-blocked analogue acetamidoglycylglycinate and the inhibitory effect of oxalic, malic, and citric acid. Triglycine, tetraglycine, and pentaglycine were also fully hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O yielding glycine as the final product of hydrolysis. K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O also exhibited hydrolytic activity toward a series of other dipeptides.