New complexes based on electrostatic interactions between the tetracobalt Dawson-derived sandwich polyanion [Co-4(H2O)(OH)(P2W15O56)(2)](17-) and the tetracationic zinc porphyrins, ZnTMpyp(4+) or ZnOETPyP4+, are formed in aqueous solutions. In these complexes, porphyrins can be excited by visible light and then play the role of photosensitizers able to give electrons to polyoxometalate. Consequently, these complexes exhibit photocatalytic behaviour in the visible domain. Indeed, the photocatalytic reduction of silver ions in the presence of these complexes and propan-2-ol is observed in aerated and deaerated aqueous solutions. The formed silver nanoparticles are stable in air conditions without illumination. (C) 2009 Elsevier B.V. All rights reserved.