Isolation of a soluble [NHex(4)](+) salt has allowed a detailed electrochemical study of the anion alpha-[SMo12O40](2-) to be undertaken. Four reversible one-electron-reduction processes are observed in CH2Cl2 solution. Controlled potential electrolysis led to isolation of tetraalkylammonium salts of the one-electron-reduced anion alpha-[SMo12O40](3-) and the two-electron-reduced anion alpha-[SMo12O40].(4-) [SMo12O40](3-) is Stable to disproportionation in dry solvents (K-dis = 10(-7.4)). EPR and magnetic susceptibility data indicate that [SMo12O40](3-) is a Simple paramagnet (S = 1/2) while [SMo12O40](4-) is paramagnetic with the mu (eff) values decreasing at low temperatures. Solutions of the two-electron-reduced species are EPR silent, but microcrystalline powders show very weak signals. The crystal structure of alpha-[NBu4](3)[SMo12O40] has been determined (triclinic P1; a = 13.840(3) Angstrom; b = 15.587(4) Angstrom; c = 19.370(3) Angstrom; alpha = 94.82(2)degrees; beta = 93.10(1)degrees; gamma = 91.05(2)degrees; Z = 2). There is disorder around the C-2 axis of the central SO42- tetrahedron. In the presence of aqueous HClO4 (0.045 M) in thf/H2O or MeCN/H2O (98/2 v/v), [SMo12O40](2-) exhibits five two-electron-reduction processes. Under these conditions. [SMo12O40](3-) protonates and disproportionates into [SMo12O40](2-) and the (2e(-), 2H(+))-reduced anion [H2SMo12O40](2-).