novel molybdenum(VI) arsenates, Cs5Mo8O24(OH)(2)AsO4 . 2H(2)O and Cs7Mo8O26AsO4, have been prepared and their structures determined. Cs5Mo8O24(OH)(2)AsO4 . 2H(2)O was synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric analysis. Diffraction measurements were performed on a CCD area detector system. It crystallizes in the orthorhombic space group Cmcm with a = 8.8048(3) Angstrom, b = 23.4314(9) Angstrom, c = 16.0499(6) Angstrom, and Z = 4. The structure consists of [AsMo8O30H2](5-) cluster anions which are made up of two tetranuclear (Mo4O15H) cores linked by an arsenic(V) atom. Upon heating, half of the structural unit first transforms to Cs7Mo8O26AsO4, and the other half decomposes to Cs3AsO4 and MoO3 at higher temperature. Cs7Mo8O26AsO4 crystallizes in the orthorhombic space group Ibca with a = 12.1247(4) Angstrom, b = 22.9153(7) Angstrom, c = 24.3777(7) Angstrom, and Z = 8. It possesses [AsMo8O30](7-) cluster anions resembling to those found in the original structure. Cs7Mo8O26AsO4 may be viewed as a dehydrated form of Cs5Mo8O24(OH)(2)AsO4 . 2H(2)O from which the lability of [AsMo8O30H2](5-) with respect to dissociation of AsO43- is first shown in the solid state. Structural differences between [AsMo8O30H2](5-) and [AsMo8O30](7-) are closely correlated with the cesium cations and water molecules present in the crystals. The title compounds are also the first structurally characterized examples in the Cs-Mo-As-O system.