The inhibitory effects of vanadium(V) were determined on the oxidation of glycerol 3-phosphate (G3P) catalyzed by glycerol-3-phosphate dehydrogenase (G3PDH), an enzyme with a thiol group in the active site. G3PDH from rabbit muscle was inhibited by vanadate, and the active inhibiting species were found to be the vanadate dimer and/or tetramer. The dimer was a sufficiently weak inhibitor at pH 7.4 with respect to G3P; the tetramer could account for all the observed inhibition. The tetramer was a competitive inhibitor with respect to G3P with a Ki of 0.12 mM. Both the dimer and tetramer were noncompetitive inhibitors at pH 7.4 with respect to NAD with Ki's of 0.36 mM and 0.67 mM. G3PDH inhibited by vanadate was reactivated when EDTA complexed the vanadate. The reactivation occurred even after extended periods of incubation of G3PDH and vanadate, suggesting that the inhibition is reversible despite the thiol group in the active site. Analogous reactivation is also observed with glyceraldehyde-3-phosphate dehydrogenase (Gly3PDH). Gly3PDH is an enzyme that previously had been reported to undergo redox chemistry with vanadate. The work described in this paper suggests vanadate will not necessarily undergo redox chemistry with enzymes containing thiol groups exposed on the surface of the protein.