Reaction of in situ generated copper(II)-monosubstituted Keggin polyoxometalates and copper(II)-phenanthroline-oxalato complexes in ammonium or rubidium acetate buffers led to the formation of the hybrid inorganic-metalorganic compounds E-4[Cu(phen)(H2O)(4)](2)[Cu-4(phen)(4)(H2O)(4)(ox)(3)](0.6)[Cu-2(phen)(2 )(H2O)(4)(ox)](0.4)[Cu(phen)(ox)](0.8)[{SiW11O39Cu(H2O)}(2){Cu-2(phen)(2 )(ox)}]center dot 20H(2)O [E: Rb (1), NH4 (2)]. The two compounds have been characterized by means of elemental analysis, thermogravimetry, infrared and electron paramagnetic resonance spectroscopies, and magnetic susceptibility measurements, and their structures have been established by single-crystal X-ray diffraction. Both compounds are isostructural, and they contain a discrete bimolecular hybrid polyanion and several types of copper-phenanthroline complexes of variable nuclearity. The main structural features of these compounds are the presence of the new hybrid POM [{SiW11O39Cu(H2O)}(2){Cu-2(phen)(2)(mu-ox)}](10-), where the dinuclear copper-oxalato complex is sandwiched by two copper-monosubstituted POMs, and the coexistence of five different copper-phenanthroline species with nuclearities ranging from one to four.