Reaction of Na7H[Nb6O19]15H(2)O with NaVO(3)2H(2)O at 220 degrees C in the presence of NaHCO3 gives new bicapped alpha-Keggin vanadododecaniobate [VNb12O40{NbO(CO3)}(2)](13-), isolated and structurally characterized as Na9H4[VNb12O40{NbO(CO3)}(2)]37H(2)O (1). According to V-51 NMR and ESI-MS data, this anion equilibrates in solution with [VNb12O40](15) and oligomeric species that result from dissociation of the {NbO(CO3)}(+) fragments. In the presence of potassium, the same reaction gives [VxNb24O76](n-) (x = 4, n = 12 (2a); x = 3, n = 17 (2b)). The anions with x = 3 and 4 cocrystallize together, but exist as separate entities both in solid and in solution according to V-51 MAS NMR and ESI-MS data.