High-resolution x-ray diffraction data were collected from 10 to 433 K on a homogeneous polycrystalline specimen of Mg0.9Fe0.1SiO3-perovskite. Rietveld structural refinement and x-ray absorption fluorescence measurements demonstrate that Fe substitutes for Mg and not Si. The thermal expansion behavior is anisotropic, with the orthorhombic distortion becoming less with increasing temperature. The volumetric thermal expansion, which is dominated by the decrease in octahedral tilts, is 1.9¿10-5 K-1 between 150 and 373 K. No evidence of conversion to enstatite could be found up to 873 K. ¿ American Geophysical Union 1990