Experimental data on the partitioning of Fe and Mg between coexisting silicate perovskite, magnesiowustite, and gamma-(Mg,Fe)(2)SiO4 demonstrate that Fe substitution in perovskite is strongly coupled to Al2O3 concentration. In Al2O3-free compositions, perovskite has a low Fe/(Fe+Mg) ratio : for a bulk Fe/(Fe+Mg) of 0.11, perovskite has a value close to 0.04, whereas magnesiowustite has a ratio near 0.17. In peridotitic mantle, however, where the perovskite should contain 4 to 5 weight percent Al2O3, it has essentially the same Fe/(Fe+Mg) ratio as coexisting magnesiowustite. Under lower mantle conditions, therefore, perovskite and magnesiowustite should, in peridotite, each have an Fe/(Fe+Mg) ratio close to 0.11.