The compressional and shear wave velocities in a series of polycrystal samples of magnesiowustite (Mgx, Fe1-x)YO have been measured using the pulse superposition method. Each specimen was characterized in terms of composition, stoichiometry, and porosity by chemical and microprobe analysis, and by x ray diffraction. In addition, the measurement of velocity in three perpendicular directions has demonstrated that the individual specimens were isotropic to within 1/4%. The application of empirical corrections to account for the effects of nonstoichiometry and porosity has allowed the determination of the isotropic elastic properties for the entire stoichiometric (Mg, Fe)O solid solution series. Our results for MgO (KS = 1619¿16 kbar, μ = 1304¿16 kbar) are consistent with previous determinations of these properties from single-crystal measurements. In the case of FeO (KS = 1835¿31 kbar, μ = 589¿14 kbar), our data generally correspond to the results of previous investigations, which used both ultrasonic and static compression techniques, when corrections are applied for the effects of nonstoichiometry. Our measurements on Fe-rich magnesiowustites have demonstrated clearly that the isotropic elastic properties, especially the bulk modulus KS, are strongly dependent on the defect structure (nonstoichiometry) of the specimens. The increasing number of vacancy clusters toward the FeYO end-member correlates with decreasing bulk modulus and anomalously high values of (∂K/∂P)T. However, when corrections are applied for the nonstoichiometric effects, the elastic properties of the (Mg, Fe)O solid solution series behave according to the linear compliance relation proposed by Liu (1968) and Jackson et al. (1978).