We describe the reduction of vanadium (V) to vanadium (IV) in cultures of Thiobacillus thiooxidans on elemental sulfur, for initial vanadium (V) concentrations up to 5 mM. The vanadium (V) is reduced by intermediate compounds generated by bacterial oxidation of elemental sulfur. The limit of initial vanadium (V) allowing bacterial action seems to be related to the inhibition caused by such vanadium species, rather than connected to the vanadium (IV) species, which did not show inhibitory effects up to concentrations of about 0.1 M. This reduction mechanism of vanadium (V) is potentially applicable in the recovery of vanadium from spent solid catalysts, by a low-cost methodology.