A second oxidation state of the Mo3CuS4 cuboidal cluster has been prepared for the first time by reacting Cu+ or solid CuCl with the trinuclear Mo-3(IV) incomplete cuboidal cluster [Mo3S4(H2O)(9)](4+) in aqueous acidic solutions and is here characterized as [Mo3CuS4(H2O)(10)](5+). The previously prepared 4+ cube, which crystallizes from 4 M Hpts (pts(-) = p-toluenesulfonate) as the edge-linked double cube [{Mo3CuS4(H2O)(9)}(2)](pts)(8).20H(2)O, behaves as a single cube on elution from a cation-exchange resin with HCl or Hpts, and is assigned the corresponding [Mo3CuS4(H2O)(10)](4+) (or related) formula as the dominant species in solutions here studied. Furthermore EPR spectra for [Mo3CuS4(H2O)(10)](4+) in 2 M HCl or 2 M Hpts are similar and provide evidence for an unpaired electron interacting with a single Cu nucleus. An absorption peak in the near-IR at 975 nm for Hpts solutions only may be assigned to the presence of some double cube. At wavelengths in the W-vis range, 2 M Hpts solutions obey Beer’s law, and the expression for complexing with Cl- gives K = 3.5 x 10(3) M(-1) consistent with the presence of a single cube. At 25 degrees C kinetic studies on the addition of Cu+ to [Mo3S4(H2O)(9)](4+) indicate a [Cu+]-dependent process rate constant of 980 M(-1) s(-1) at I = 2.00 M (LiClO4). Kinetic studies are also reported for the 2:1 hexaaquairon(III) and [Co(dipic)(2)](-) (dipic = pyridine-2,6-dicarboxylate) oxidations of [Mo3CuS4(H2O)(10)](4+), which occur in two stages with intermediate formation of [Mo3CuS4(H2O)](5+). The second of the rate constants is reproduced in separate studies on [Mo3CuS4(H2O)(10)](5+). Studies on the [Ru(NH3)(5)(H2O)](2+) reduction of [Mo3CuS4(H2O)(10)](5+) are also reported. As a result of this work an electron-transfer mechanism is proposed for reactions leading to the Cu2+ displacement of M(2+) from [Mo(3)MS(4)(H2O)(10)](4+) cubes; e.g., M = Fe and Ni.