Experimental studies of Fe_1−xO wüstite and (Mg_0.8Fe_0.2)O ferropericlase were performed at room temperature and high pressure using a diamond anvil cell technique. Mössbauer spectroscopy indicated a magnetic ordering transition in FeO at 4.7 GPa, while no evidence of magnetic ordering was detected in ferropericlase up to at least 50 GPa. A structural cubic-to-rhombohedral distortion was observed in both materials using powder X-ray diffraction: at 12 GPa for FeO and at 35 GPa for ferroperi-clase. This suggests that the structural distortion is independent of magnetic ordering in the FeO-MgO system. A complete single-crystal elasticity...