A novel compound containing a hexacalcium duster cation, one carbonate anion, and one calcium cation assembled on a trimeric trititanium(W)-substituted Wells-Dawson polyoxometalate (POM), [{Ca-6(CO3)(mu(3)-OH)(OH2)(18)}(P2W15Ti3O61)(3)Ca-(OH2)(19-) (Ca(7)Ti(9)Trimer), was obtained as the Na7Ca6 salt (NaCa-Ca7Ti9Trimer) by the reaction of calcium chloride with the monomeric-trititanium(1V)-substituted Wells-Dawson POM species "[P2W15Ti3O59(OH3)(9-)" (Ti(3)Monomer). Ti(3)Mononier was generated in situ under basic conditions from the separately preared tetrarneric species with bridgin Ti(OH2)(3) groups and an encapsulated Cl- ion, [{P2W15Ti3O59)(3)(OH3)(4){mu(3)-Ti-(H2O)(3)}(4)Cl](21-) (Ti(16)Tetranter): The Na7Ca6 salt of Ca(7)Ti(9)Trimer was characterized by complete-elemental analysis, thermogravimetric (TG) and differential thermal analyses (DTA), FTIR, single-crystal X-ray structure analysis, and solution W-183 and P-31 NMR spectroscopy. X-ray crystallography revealed that the [Ca-6(CO3)(mu(3)-OH)(OH2)(18)](9+) cluster cation was composed of six calcium cations linked-by one mu(6)-carborrato anion and one mu(3)-OH- anion. The duster cation was assembled, together with one calcium ion, on a trirneric species composed of three tri-Ti(IV)-substituted Wells-Dawson subunits linked by Ti-O-Ti bonds. Ca(7)Ti(9)Trimer is an unprecedented POM species containing an alkaline-earth-metal cluster cation and is the first example of alkaline-earth-metal ions clustered around a titanium(IV)-substituted POM.