It has been repeatedly suggested that an iron-rich oxide might accumulate on the Earth's core-mantle boundary by various processes. Recent studies showed that FeO with a rock salt (B1)-type structure undergoes pressure- and temperature-induced metallization at the Earth's lower mantle conditions. This implies similar metallization or decomposition of the lower mantle phase, (Mg,Fe)O, under high pressure-temperature conditions. We performed simultaneous X-ray diffraction and electrical conductivity measurements on (Mg Fe )O and (Mg Fe )O magnesiowüstites up to 140 GPa and 2100 K, and we examined recovered samples by means of an analytical transmission electron microscope. The experiments revealed very high electrical conductivity of the magnesiowüstite samples and their minimal temperature dependence above 85 GPa and 1300 K, yet the samples remained insulators. We also found decomposition of (Mg Fe )O into almost pure FeO and iron-rich (Mg,Fe)O due to metallization of the FeO component, while such a reaction was not observed in (Mg Fe )O. The observed high electrical conductivity and decomposition of iron-rich (Mg,Fe)O magnesiowüstite could enhance the heterogeneities in the electrical and thermal conductivity at the Earth's core-mantle boundary region. Key Points Electrical conductivity of iron-rich (Mg,Fe)O up to 140 GPa and 2100 K Highly conductive but insulating iron-rich (Mg,Fe)O in the lower mantle Decomposition of (Mg Fe )O at high P-T conditions ©2014. American Geophysical Union. All Rights Reserved. 0.20 0.80 0.05 0.95 0.05 0.95 0.20 0.80 0.05 0.95