Three new molybdenum(VI) arsenate hydrates, Cs2Mo2O5As2O7. H2O (1), Cs2Mo2O5(HAsO4)(2). H2O (2), and Cs-4-Mo6O18(H2O)(HAsO4)(2). 2.5H(2)O (3), have been prepared and structurally characterized by single-crystal X-ray diffraction, thermal analysis, and IR spectroscopy. Crystal data : 1, monoclinic, P2(1)/c a = 7.9783(5) Angstrom, b = 10.7837(6) Angstrom, c = 15.5213(9) Angstrom, beta = 93.594(1)degrees and Z = 4; 2, triclinic, P (1) over bar, a = 7.8165(2) Angstrom, b = 9.8902(3) Angstrom, c = 11.0401(3) Angstrom, alpha = 63.814(1)degrees, beta = 70.475(1)degrees, gamma = 72.655(1)degrees, Z = 2; 3, monoclinic, P2(1)/n, a = 8.0345(1) Angstrom, b = 20.0364(4) Angstrom, c = 19.5941(3) Angstrom, beta = 92.296(1)9 Z = 4. Crystal 1 adopts an open framework structure which contains vertex-sharing bioctahedral Mo2O11 units linked by As2O74- groups to form intersecting tunnels where Cs+ cations and water molecules reside. Crystals 2 and 3 are composed of the discrete cluster anions, [Mo4O10(HAsO4)(4)](4-) and [Mo6O18(H2O)(HAsO4)(2)](4-), respectively. These anions are held in space via the interactions with Cs+ cations and hydrogen bonds. Thermal studies showed both crystals 2 and 3 are transformed into 1 upon prolonged heating. It exhibits the first experimental evidence that a metal diarsenate framework can be converted thermally from hydrogen arsenate-containing polyanions. The three compounds are the first members prepared in the Cs/Mo-VI/As-V/O system.