The effect of small cations such as H+, Li+ and Na+ on the voltammetric behavior of alpha-Keggin-type [SiMo12O40](4-) and [PMo12O40](3-) complexes was investigated in CH3COCH3 and CH3CN. For the [SiMo12O40](4-) complex, the presence of Li+ or Na+ caused the one-electron waves to be converted into a two-electron wave at ca. 0.3 V more positive than the first one-electron wave. In the presence of Li+ or Na+, the [PMo12O40](3-) complex underwent a two-electron reduction at the same potential as the original first one-electron wave in CH3COCH3, whereas it exhibited only successive one-electron waves in CH3CN. The addition of a trace amount of H+ produced new two-electron waves at more positive potentials. These findings give a clue to the understanding of the reactivity of polyoxometalates as redox catalysts.