The static compression of two magnesiow¿stites with 60 and 80 mol % FeO has been measured up to 500 kbar under quasi-hydrostatic conditions. Within the framework of a Birch-Murnaghan equation of state, these and other data for magnesiow¿stites are consistent with linear variations of the 1-bar isothermal bulk modulus and of its 1-bar pressure derivative, from pure w¿stite (K0=1.545¿0.01 Mbar, K0'=2.75¿0.3) to periclase (K0=1.600¿0.005 Mbar, K0'=4.85¿0.2). The composition with 80% FeO, but not that with 60% FeO, exhibits a second-order like phase change analogous to the magnetic transition already described for w¿stite at around 150 kbar. The lack of a significant dependence of the bulk moduli on nonstoichiometry suggests that the effect of pressure on the defect structure is not as simple as assumed previously. ¿ American Geophysical Union 1989