1-n-Butyl-3-methylimidazolium (bmim) salts of [alpha-SiM12O40](4-) and [alpha-S2M18O62](4-) (M = Mo, W) were synthesized and characterized. In sharp contrast to the multiple one-electron diffusion-controlled reduction processes expected in solution at a glassy carbon electrode, reduction of these salts beyond the one-electron level in CH3CN (0.1 M [bmim][PF6]) led to rapid precipitation of arrays of microdroplets or microcrystals onto the surface. Upon oxidation these arrays dissolved on the electrochemical time scale, resulting in voltammograms displaying both diffusion-controlled and surface-confined behavior. These novel observations appear to be related to the ability of the [bmim](+) cations and polyoxometalate anions to form ion clusters in solution and to the lower solubilities of the reduced salts.