The trinuclear Mo-3(IV) incomplete cuboidal cluster [Mo3S4(H2O)(9)](4+) in 2 M HCl reacts rapidly (green to intense blue-green color change) with AsO2- or HAsO42- in the presence of H3PO2 as reductant to give the new air-sensitive corner-shared double. cube [Mo6As8(H2O)(18)](8+) Purification by Dowex 50W-X2 cation-exchange chromatography requires 4 M H(pts) (pts = p-toluenesulfonic acid) for elution, consistent with a high charge. The X-ray structure has been determined for crystals obtained from 4 M H(pts) solution, full formula [MO6AsS8(H2O)(18)](pts)(8). 28H(2)O. The Mo6AsS8 core structure was confirmed from ICP elemental analysis for Mo, As, and S (6:1:8) and the 8+ charge from the 3:1 stoichiometries with [Co(dipic)(2)](-) and [Fe(H2O)(6)](3+) as oxidant. Rate constants for the oxidation of [Mo6AsS8(H2O)(18)](8+) With [Co(dipic)(2)](-) have been determined and indicate strong reduction properties as compared to other heteroatom clusters. The existence of short (bonding) Mo-Mo separations, 2.716 Angstrom average, and long (nonbonding) Mo-As separations at 3.554 A, together with similar features for other heteroatom clusters, has led to a classification of the clusters into different categories. The nine 8+ double cubes, most of which contain a main group heteroatom M, are classified as (Mo3S44+)2 MO. With retention of two metal-metal bonded Mo3S44+ units. The single cubes, on the other hand, can be classified as (Mo3S44+M0), with M a transition metal, or Mo(3)S(4)(4+)rM(x) With M a main group element (x depends on group involved).