Phase equilibria in the "FeO"-V2O3 system from 1273 to 1808 K and in the range of oxygen partial pressure from 10(-15) to 10(-4) atm are investigated. High-temperature quenching, XRD, SEM-EDS, and DSC are used to determine the phase relations. Stable regions of (FeO)(s.s.), (V2O3)(s.s.), and spinel phases are considerably effected by the oxygen partial pressure, and structural models are proposed as (Fe2+, Fe3+, V2+)(1-)O-x, (V2+, V3+, V4+, Fe3+)(2)O-3+(x), and (Fe2+, Fe3+, V3+)(Fe2+, Fe3+, V3+, Va)(2)O-4. Continuous solid solution FeV2O4-Fe3O4 is formed. The nonstoichiometry of FeV2O4 is attributed to the appearance of vanadium vacancies for electroneutrality due to the oxidation of Fe2+. The standard Gibbs energy of formation for FeV2O4 and component activities in FeV2O4-Fe3O4 solid solution at 1623 and 1773 K are derived based on the equilibrium oxygen partial pressure. The cation distribution in FeV2O4 at different temperatures is obtained according to site preference energy.