The electrochemical behavior of K2S2O7-KHSO4-V2O5, K2S2O7-V2O4 and K2S2O7-KHSO4-V2O4 melts was studied in argon and SO2/air atmospheres using a gold electrode. In order to identify the voltammetric waves due to KHSO4, molten KHSO4 and mixtures of K2S2O7-KHSO4 were investigated by voltammetry performed with Au and Pt electrodes in an argon atmosphere. It was shown that H+ reduction took place at 0.26 V vs. an Ag+/Ag reference electrode, i.e., at a potential in between the V(V) --> V(IV) and V(IV) --> V(III) reduction stages. The presence of KHSO4 caused an increased concentration of V(III) species in the V2O5 containing molten electrolytes. This effect may be caused either by protonic promotion of the V(IV) --> V(III) reduction (VO2+ + 2H(+) + e(-) --> V3+ + H2O) or by chemical reduction of V(IV) complexes with hydrogen, formed from H+ as the product of the electrochemical reduction. Both the V(V) --> V(IV) reduction and the V(IV) --> V(V) oxidation remained one-electron electrochemical reactions after the addition of KHSO4 (or water) to the H2S2O7-V2O5 melt. Water had no noticeable effect on the V(V) --> V(IV) reduction but the V(IV) --> V(V) oxidation proceeded at higher polarizations in the water-containing melts in both argon and SO2/air atmospheres. This effect may be explained by participation of the water molecules in the V(IV) active complexes.