The formation processes of V(V)- and V(IV)-substituted molybdophosphate complexes with the Keggin structure were studied by a combined redox and ion-transfer voltammetric study. There was a big difference in the formation processes between V(V)and V(V)-substituted mixed complexes in aqueous solution. The Mo(VI) + P(V) + V(IV) system produced a blue mixed complex in which the peripheral Mo atoms possessed the oxidation number of VI; only a mono-substituted complex was formed even in the presence of excess of V(IV). In the Mo(VI) + P(V) + V(V) system, on the other hand, evidence was obtained for the spontaneous transformation of a mono-substituted complex into mixed complexes with higher V(V) contents.