For the [(P2O7)Mo18O54](4-) complex, the presence of small cations such as H+, Li+ and Na+ caused one-electron waves to be converted into four- and two-electron waves in a complex manner. With the addition of a trace amount of H+, a four-electron reduction wave was obtained in solvents of weaker basicity like acetone, acetonitrile and propylene carbonate (PC); the relative permittivity did not affect the appearance of the four-electron wave. On the other hand, two-electron waves were obtained in solvents of stronger basicity like N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and N-methylpyrrolidinone (NMP). With the addition of Li+ or Na+, the one-electron waves were converted into two-electron waves only in acetone, indicating that the conversion can occur in solvents of both weak basicity and low relative permittivity.