Coulometric titration, an electrochemical method for measuring oxidation isotherms, has been used to characterize the redox properties of V2O5 and Mg-3(VO4)(2) between 823 and 973 K. V2O5 shows distinct regions in the isotherms corresponding to equilibrium with mixtures of V2O3 and V2O4 and of V2O4 and V2O5. From this data, the enthalpies for oxidation of V2O3 to V2O4 and for V2O4 to V2O5 are shown to be -380 +/-10 and -285 +/- 20 kJ mol(-1) O-2, respectively. Oxidation isotherms for Mg-3(VO4)(2) exhibit a single step between the oxidized sample (all V+5) and a completely reduced sample (all V+3). The enthalpy of oxidation is found to increase with the oxidation state of the sample, from -370 +/- 30 kJ mol(-1) O-2 at an O:V ratio of 1.5 to -460 +/- 10 kJ mol(-1) O-2 at an O:V ratio of 2.5.