Uracil photooxygenation occurs in acidic water (pH = 1) at 25 °C, under oxygen (1 atm), irradiating with λ > 300 nm in the presence of selected polyoxometalates (POM). A marked diversity of photocatalytic behavior is registered for different POMs in terms of oxidation rate and selectivity. H3PW12O40 (PW12) appears to be the most reactive photocatalyst, by far superior to isostructural complexes, leading to a product distribution typical of OH· dominated oxidations, while Na4W10O32 (W10) and Na12[WZn3(H2O)2(ZnW9O34)2] (Zn5W19) exhibit a preferential reactivity towards uracil glycol. Kinetic studies and radical scavenger probes, performed on target intermediates and model diols, highlight a substantial difference in the mechanism of photocatalysis by the three complexes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)