The first examples of polyoxometalates incorporating uranyl hetero groups have been synthesized by reaction of Na-9[A-PW9O34] with UO2(NO3)(2). Sodium, ammonium, and potassium salts of [M-2(UO2)(2)-(PW9O34)(2)](12-) (M = Na, NH4, K) were characterized by P and W NMR spectroscopy and single-crystal structural analysis. The sodium salt (1) contains an anion of C-i symmetry in which two PW9O34 groups sandwich two UO22+ and two Na+ cations. The uranium atoms have pentagonal-bipyramidal coordination, achieved by three equatorial bonds to one PW9O34 and two bonds to the other. The sodium cations have approximate tetrahedral coordination geometry. Single-line P and five-line W NMR spectra confirm that the anion structure is maintained in solutions containing excess Na+. The ammonium and potassium salts contain anions in which the UO22+ cations (pentagonal bipyramids) are unsymmetrically sandwiched between the PW9O34 groups, as are two closely associated cations, one internal and one external. In solution both, salts give two-line P and nine-line W NMR spectra that are consistent with the solid-state structures. Solution equilibria are rapidly established between the two structure types and are dependent upon the concentrations and identities of the cations. Based on integrated P NMR spectra, the equilibrium constants for [Na-2(UO2)(2)(PW9O34)(2)](12-) + 2Me(+) <-> [M-2(UO2)(2)(PW9O34)(2)](12-) + 2Na(+) are 128 +/- 12 (M = K+) and 1.8 +/- 0.8 (M = NH4+). The requirement of the second cation in order to satisfy the mass-action expression for K-eq, coupled with a W NMR spectrum that implies C-s symmetry, demonstrates that the external ion-paired cation undergoes rapid exchange between several surface sites of the heteropolyanion. Solutions of 1 are stable at pH 7 in the presence of excess sodium cations, and the anion is transferable into toluene by phase-transfer techniques. Addition of calcium ions to solutions of 1 generates a new one-line P NMR spectrum of a Ca derivative which is assumed to be isostructural with 1.