In 1999, a catechol dioxygenase derived from a V-polyoxometalate was reported which was able to perform a record > 100 000 total turnovers of 3,5-di-tert-butylcatechol oxygenation using O-2 as the oxidant (Weiner, H.; Finke, R. G. J. Am. Chem. Soc. 1999, 121, 9831). An important goal is to better understand this and other vanadium-based catechol dioxygenases. Scrutiny of 11 literature reports of vanadium-based catechol dioxygenases yielded the insight that they all proceed with closely similar selectivities. This, in turn, led to a "common catalyst hypothesis" for the broad range of vanadium based catechol dioxygenase precatalysts presently known. The following three classes of V-based compounds, 10 complexes total, have been explored to test the common catalyst hypothesis: (i) six vanadium-based polyoxometalate precatalysts, (n-Bu4N)(4)H5PV14O42, (n-Bu4N)(7)SiW9V3O40, (n-Bu4N)(5)[(CH3CN)(x)Fe-II-SiW9V3O40], (n-Bu4N)(9)P2W15V3O62, (n-Bu4N)(5)Na-2[(CH3CN)(x)Fe(II)center dot P2W15V3O62], and (n-Bu4N)(4)H-2-gamma-SiW10V2O40; (ii) three vanadium catecholate complexes, [(VO)-O-V(DBSQ)(DTBC)](2), [Et3NH](2)[(VO)-O-IV(DBTC)(2)]center dot 2CH(3)OH, and [Na(CH3OH)(2)](2)[V-V(DTBC)(3)](2)center dot 4CH(3)OH (where DBSQ = 3,5-di-tert-butylsemiquinone anion and DTBC = 3,5-di-tert-butylcatecholate dianion), and (iii) simple VO(acac)(2). Product selectivity studies, catalytic lifetime tests, electron paramagnetic resonance spectroscopy (EPR), negative ion mode electrospray ionization-mass spectrometry (negative ion ESI-MS), and kinetic studies provided compelling evidence for a common catalyst or catalyst resting state, namely, Pierpont's structurally characterized vanadyl semiquinone catecholate dinner complex, [VO(DBSQ)(DTBC)](2), formed from V-leaching from the precatalysts. The results provide a considerable simplification and unification of a previously disparate literature of V-based catechol dioxygenases.