Hydrolytic cleavage: of 4-nitrophenyl phosphate (NPP), a commonly used DNA model substrate, was examined in the presence of series of lanthanide-substituted Keggin-type polyoxometalates (POMs) [Me2NH2](11)-[Ce-III(PW11O39)(2)], [Me2NH2](10) [Ce-IV(PW11O39)(2)] (abbreviated as (Ce-IV(PW11)(2)), and, K-4[EuPW11O39] by means of NMR and luminescence spectroscopies and density functional theory (DFT) calculations. Among the examined complexes, the Ce(W)-substituted Keggin POM (Ce-IV(PW11)(2)) showed the highest reactivity, and its aqueous speciation was fully determined under different conditions of pD, temperature, concentration, and ionic strength by means of (31)p and P-31 diffusion-ordered NMR spectroscopy. The cleavage of the phosphoester bond of NPP in the presence of (Ce-IV(PW11)(2)) proceeded with an observed rate constant k(obs) = (5.31 +/- 0.06) X 10(-6) s(-1) at pD.6.4 and 50 degrees C. The-pD dependence of NPP hydrolysis exhibits a bell-shaped profile, with the fastest rate observed at pD 6.4. The formation constant (K-f = 127 M-1) and catalytic rate constant (k(c), = 19.41 X 10(-5) s(-1)) for the NPP-Ce(IV)-Keggin POM complex were calculated, and binding between (Ce-IV(PW11)(2)) and the phosphate group of NPP was also evidenced by the change of the chemical shift of the P-31 nucleus in NPP upon addition of the POM complex. DFT calculations revealed that binding of NPP to the parent catalyst (Ce-IV(PW11)(2)) is thermodynamically unlikely. On the contrary, formation of complexes Ce-IV(PW11, is considered to be more favorable, and the most stable complex, with the monomeric 1:1 species, (Ce-IV(PW11), is considered to be more favorable, and the most stable complex, (Ce-IV(PW11)(H2O)(2)(NPP-kappa O)2](7-), was found to involve two NPP ligands coordinated to the Ce-IV center of (CePW11)-P-IV in the monodentate fashion. The formation of such species is considered to be responsible for the hydrolytic activity of Ce-IV(PW11)(2) toward phosphomonoesters. On the basis of these findings a principle mechanism for the hydrolysis of NPP by the POM is proposed.