A novel type of heteropolyoxoanion (HPOA) precursors [[Ca (H(2)O)](6)[P(4)M(6)O(34)](2)](12-) [M = W(VI) (1), Mo(VI) (2)] has been synthesized and structurally characterized, which represents the rare membership of the HPOA family constituted by two early-transition-metal clusters ([P(4)M(6)O(34)](12-)) and a main-group metal cluster of high nuclearity ([Ca(6)](12+)).