The voltammetric behavior of several Keggin-type heteropolymolybdate anions, [XMo12O40]n- (X = S, P, As, V, Si, Ge; n = 2-4) at a glassy carbon (GC) electrode was investigated in acetone or acetonitrile. The one-electron reduction waves for the Keggin anions were converted into two-electron waves as a function of acid concentration. The effect of protons on the successive reduction steps was verified experimentally, and the reduction mechanism of the conversion process was elucidated.