The phase boundaries of w¿stite at high pressure are calculated using a thermodynamic data set describing the Fe-O system. A new thermodynamic description of w¿stite is presented, whereas the thermodynamic descriptions of iron, oxygen, magnetite, and hematite are taken from literature. An equation for the Gibbs energy of formation of w¿stite at ambient pressure rests on recent calorimetric studies. The description gives reasonable values for the eutectoidal point and the phase boundaries of w¿stite at high temperatures and furthermore explains the formation of a metastable intermediate two-phase mixture of nearly stoichiometric w¿stite and magnetite on heating quenched w¿stite to around 500 K. Extrapolations to high pressure are made by taking the compositional, temperature, and pressure variation of the molar volume of w¿stite into consideration. In view of the spread in the experimental and estimated bulk modulus values for w¿stite, three alternatives regarding the compositional variation of the bulk modulus are considered. The assumption of a composition independent bulk modulus (K0=180 or 150 GPa) results in stoichiometric FeO being the stable phase at high pressures, whereas the assumption of a composition dependent bulk modulus results in a significant widening of the homogeneity region of w¿stite and also indicates that FeO disproportionates to iron and nonstoichiometric w¿stite at high pressures. The effect of changes in ∂K0/∂T and ∂K0/∂P on the calculated phase boundaries is considered. Earlier experimental studies and theoretical considerations are discussed as are the implications of phase transitions in w¿stite and in the neighboring phases. ¿ American Geophysical Union 1996