The recently discovered area of water-soluble noble-metalbased polyoxometalates (POMs) [1a] is currently developing fast, not only because of their interesting structures and properties but also because of the promising applications of these compounds. [1][2][3][4][5] Currently polyoxopalladates(II) are the noble-metalbased polyanion family comprising the largest number of members.In Figure 1). This polyanion contains the largest number of palladium ions yet found in polyoxopalladate chemistry. The bonding of each Cu II ion involves the rare eightfold oxo-coordination. Polyanion 1 has been prepared by heating of Pd 3 (CH 3 COO) 6 and Cu-(CH 3 COO) 2 ·H 2 O in sodium phosphate buffer (0.5 m, pH 6.9), and isolated as a hydrated sodium salt, Na 20 [Cu 2 Pd 22 P 12 O 60 (OH) 8 ]·58 H 2 O (Na-1), which is stable in the solid state under air and light, is soluble in water, and can be repeatedly recrystallized from aqueous solution.The pH value of the reaction mixture is a crucial factor for the formation of Na-1. For example, at slightly lower pH values (6.0-6.2), we observed the formation of dark-red, octahedral crystals of a hydrated sodium salt of a compound with a {CuPd 12 P 8 } core, [5] the crystal structure and properties of which will be presented elsewhere. At the intermediate pH range 6.2-6.9, a mixture of the two compounds was obtained. In the absence of copper(II) ions, a similar synthetic