The photocatalytic behaviours of the Dawson salt alpha-K-6[P2W18O62] and two isomers of the tetracobalt Dawson-derived sandwich complexes, of alpha beta beta alpha-Na-17[Co-4(H2O)(OH)(P2W15O56)(2)]center dot 51H(2)O center dot 2NaCl and alpha alpha beta alpha-Na-16[Co-4(H2O)(2)(P2W15O56)(2)]center dot 51H(2)O (abbreviated beta beta-{Co4P4W30} and alpha beta-{Co4P4W30}, respectively), are described and compared. The direct photochemical excitation of the polyoxometalates (POM) in the presence of propan-2-ol as electron donor leads to their reduction. With polyoxometalates as photocatalyst and propan-2-ol as sacrificial electron donor, the reduction of (Ag2SO4)-S-1 from aqueous solutions is observed leading to metallic Ag-n(0) clusters and colloidal metal nanoparticles stabilized by POM. In the case of both {Co4P4W30}, TEM experiments reveal that most of the Ag-n particles obtained with a slight excess of Ag+ are spherical with a quite large distribution in size between 10 and 100 nm. (C) 2008 Elsevier B.V. All rights reserved.