The Keggin-type beta-[PW12O40](3-) complex was prepared, and the voltammetric behavior was compared with that of the Keggin-type alpha-[PW12O40](3-) complex. Both complexes underwent a four-step one-electron reduction in CH3CN where no protonation accompanied reduction. With the addition of CF3SO3H, the one-electron reduction waves were converted into two two-electron waves, followed by an ill-defined wave. The addition of water to the CH3CN + CF3SO3H system caused the first two-electron wave to split into two one-electron waves. The effect of acid or water on the voltammetric behavior is discussed also.