A thermogravimetric analysis (TGA)/SO3 equilibrium technique was used to study the reaction of CeO2 at 1073 K with NaVO3 in equilibrium with SO3 over the partial pressure range of 10(-7) to 10(-3) atm in air. The CeO2 was inert to chemical reaction for SO3 partial pressures up to 3 x 10(-6) atm, where reaction of CeO2 with the V2O5 component of the melt commenced with CeVO4 being produced. Thermodynamic calculations from the TGA/SO3 data are consistent with a standard Gibbs energy of formation for CeVO4 at 1073 K of about - 1445 kJ/mol, and an activity coefficient for V2O5 in the melt of about 0.01. The results are discussed in terms of vanadate-sulfate melt thermochemistry and earlier findings in the field.