The redox properties of a vanadium phosphorus oxide (VPO) catalyst with a V:P ratio of one were investigated using Coulometric Titration at 873 K. Equilibrium between (VO)(2)P2O7 and VOPO4 exists at a P(O-2) of 3 x 10(-4) atm, corresponding to Delta G of -60 kJ/mol O-2. This value for VPO is significantly lower than that measured with other vanadium-containing catalysts that have been studied. Furthermore, compared to other vanadium catalysts, V+4 was stabilized against further reduction at lower P(O-2). These redox thermodynamics may help to explain the unique catalytic properties of VPO catalysts for partial oxidation of butane to maleic anhydride.