Reverse pulse polarographic studies of Mo(V)(2)+H2SO4 solutions showed an anodic polarographic wave due to the oxidation of a Mo(III) species generated at the mercury electrode. It is suggested that this Mo(III) species is not the well-known dimeric Mo(III) species Mo-2(OH)(2)(4+), and a new structure is proposed. Studies of the reversibility of the electrode processes, where Mo(V)(2) is reduced and Mo(III) is oxidized, were performed and it was proved that electron transfer is the rate-determining step in a sulfuric acid medium. Formation of a yellow Mo(V) species from Mo(VI) solution in a coulometric experiment occurred when the acidity was greater than 1.0 M H2SO4 and low temperatures were used. This species is also formed in the chemical reduction of either Mo(VI) or Mo(V)(2) solutions using a Jones reductor in the presence of oxidants such as oxygen.