Two new hybrid compounds constructed from Keggin type polyoxometalates and copper(II) complexes of tetradentate ligands containing amine and pyridyl groups, namely [Cu(bpmen)(H2O)][SiW12O40{Cu(bpmen)}] (1) and [SiW12O40{Cu(bpmpn)(H2O)}(2)]center dot 3H(2)O (2) (bpmen, N,N'-dimethyl-N,N'-bis-(pyridin-2-ylmethyl)-1,2-diaminoethane; bpmpn, N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-1,3-diaminopropane), have been synthesized under hydrothermal conditions and characterized by elemental analyses and infrared and Raman spectroscopy. Thermal stability of 1 and 2 has been studied by means of thermogravimetric analyses and variable temperature powder X-ray diffraction. Both compounds undergo single-crystal to single-crystal transformations promoted by reversible dehydration processes that have been followed by single-crystal X-ray diffraction. Structures of 1 and 2, and also of their corresponding anhydrous phases la and 2a, have been established. The layered structure of 1 shows rows of monodecorated polyanions with complex cations occupying intralamellar spaces, whereas trans-didecorated species in 2 lead to stacked honeycomb-like metal organic layers forming channels where Keggin clusters are accommodated. Structural differences relate to changes in the complex geometry and ligand conformation when going from bpmen to bpmpn. Dehydration of 1 promotes coordination of the complex countercation and consequent formation of a cis-didecorated species in la, whereas changes in the structure of 2a are more subtle. Structural variations upon dehydration are reflected in the electron paramagnetic resonance spectra.