Two new polyoxomolybdate-diphosphate complexes with the formula [(O(3)PXPO(3))Mo6O18(H2O)(4)](4-) (I, X = CH2; II, X = O) have been synthesized and characterized by P-31-, C-13-, H-1-NMR, IR, and elemental analysis. Anions I and II are formed under weakly acidic conditions (pH 3-3.5). An X-ray structure determination was carried out on Cs-4[(O3PCH2PO3)Mo6O18(H2O)(4)]. 7H(2)O, which crystallizes in the triclinic system, space group P (1) over bar, with a = 11.301(2) Angstrom, b = 11.499(3) Angstrom, c = 16.024(4) Angstrom, alpha = 98.43 degrees, beta = 107.92 degrees, gamma = 106.61 degrees, V = 1834.7(7) Angstrom(3), Z = 2, and rho(calcd) = 3.152 g/cm(3). Structural analysis based on 9312 independent reflections leads to a solution, R = 6.51% (1 > 2 sigma(I)). Anion I consists of a six-membered ring of MoO6 octahedra, which alternate in sharing edges and corners. The [O3PCH2PO3](4-) anion is bound with P(2) in the center of the ring and P(1) off-center acid bound above the ring to two adjacent MoO6 octahedra. Each of the remaining four molybdenums has a terminal water ligand on the same side of the ring as P(1). Anion II almost certainly has an analogous structure based on spectroscopic evidence. A second polyoxomolybdate-pyrophosphate complex with a [O3POPO3](4-) : MoO3 ratio of 1:3 has been synthesized at pH 6. This complex and anion II are possible intermediates in the molybdate-catalyzed hydrolysis of pyrophosphate which is known to be most effective at pH 2 with a secondary maximum at pH 5.5.