The relative effectiveness of a range of heteropolyoxometalate catalysts with Keggin, Dawson and transition metal-substituted lacunary structures has been compared using the homogeneous-phase oxidation of isobutyraldehyde to isobutyric acid with H2O2 in acetonitrile at 50degreesC. For the Keggin ([XM12O40](n-), X = P(V), Co(III), Fe(III), Co(II), Zn(II), M = W(VI); X = P(V), M = Mo(VI)) and Dawson ([X2Mo18O62](m-), X = P(V), S(VI)) structures, catalytic activity increased with decreasing anionic charge. In the case of the prototypical [PW12O40](3-) ion, kinetic studies indicated that there was association of the isobutyraldehyde and phosphopolyoxotungstate prior to oxidation of the former by H2O2. No fragmentation of the tungsten-containing species, such as [PW12O40](3-) and [CoW12O40](6-), occurred under the reaction conditions, as shown by P-31 NMR and UV-VIS spectroscopy, respectively while both [PMo12O40](3-) and [PMo12O40](6-) generated small amounts of peroxo-species based on P-31 NMR studies. The transition metal-substituted lacunary polyoxomolybdate systems of the type [PZ(II)(Br)Mo11O39](6-), where Z = Mn(II), Co(II), Ni(II), Cu(Il) and Zn(II), are likely degraded to (PO4[MoO(O-2)(2)](4))(3-) and/or related species by H2O2, based on UV-VIS spectroscopic studies and on P-31 NMR studies of diamagnetic [PZn(Br)Mo11O39](6-). The resulting products are only slightly more active catalysts than the Keggin anions (a maximum increase in activity by a factor of similar to7). The initial rates of reaction followed the sequence Co(II) > Ni(II) > Zn(II) > Mn(III) > Cu(II), with Mn(II) undergoing oxidation by H2O2 under the reaction conditions. The order likely corresponds to a combination of the catalytic abilities of the transition metal-substituted lacunary anions in conjunction with their rates of decomposition by H2O2, and/or the catalytic abilities of the freed transition metals and peroxo-products themselves. (C) 2002 Elsevier Science B.V. All rights reserved.