The first example of a "dimerized" Keggin anion, [{A-alpha-SiO4W9O30(OH)(3)Cr-3}(2)(OH)(3)](11-) containing low-valent heteroelements has been obtained by the condensation of [A-alpha-SiO4W9O30(OH)(3)Cr-3(OH2)3](4-) in aqueous solution. The reaction between the monomeric units was followed by UV/vis spectroscopy. The single-crystal X-ray structural analysis of (NH4)(11)[{A-alpha-SiO4W9O30(OH)(3)Cr-3}(2)(OH)(3)]. 6H(2)O . 2.5NH(4)Cl (P4(2)/nmc; a, b = 21.778(2) Angstrom, c = 21.300(2) Angstrom; Z = 4; R = 0.062 for 2961 observed independent reflections) reveals that the two Keggin anions are linked via three Cr-OH-Cr bridges. Isolation of the new polyxometalate was accomplished as NH4(+) and K+ salts, respectively, which were characterized by elemental analysis, IR and ESR spectroscopy. The bond distances and bond angles for the monomeric and dimeric species are discussed in a comparative study. The results from titration experiments of the free acids for mono-, di-, and triple chromium(III) substituted Keggin anions have been used for the description of the "dimer formation".