The voltammetric properties of the Keggin-type [XW12O40](n-) complexes were compared with those of the corresponding Mo-analogues, [XMo12O40](n-), where X = P, Ge; n = 3, 4. The one-electron waves for the Keggin anions are converted into two-electron waves by the presence of small cations such as H+, Li+ and Na+, and the conversion occurs much more easily for [XMo12O40](n-), as compared with [XW12O40](n-) with an identical ionic charge. A comparison of the redox potentials for the V(V)/V(IV) couple of the mono-vanadium derivatives made it possible to estimate the ionic radius of [XMo12O40](n-) as 5.4 Angstrom, with the ionic radius of [XW12O40](n-) = 5.6 Angstrom as reference. The big difference in voltammetric behaviors between the Mo- and W-complexes was interpreted terms of the smaller ionic radius and consequently greater basicity of [XMo12O40](3/4-), as compared with [XW12O40](3/4-).