We use atomistic computer simulation techniques to investigate the site partitioning of iron in (Mg,Fe)SiO3 perovskites. Our calculations predict that the most energetically favourable reaction for iron substitution will be a direct exchange of Fe2+ for Mg2+. Substitution of Fe into the octahedral site and Si into the 8–12 fold coordinated site, as proposed by Jackson et al. , is predicted to be extremely unlikely. Copyright 1989 by the American Geophysical Union.