The reduction of Fe(III) is one of the most important chemical changes that takes place in the development of anaerobic soils and sediments, and the reductive dissolution of iron-bearing minerals by microbes plays a critical role in this process. Despite its importance in biogeochemistry, many questions remain about the mechanism of this electron transfer reaction, in part because the speed of the fundamental chemical steps renders them inaccessible to conventional study. Ultrafast time-resolved x-ray spectroscopy is a technique that can overcome this limitation and measure changes in oxidation state and structure occurring during chemical reactions that can be initiated by a fast laser pulse. We use this approach with ~100 ps resolution to monitor the speciation of Fe atoms in maghemite nanoparticles following photo-induced electron transfer from a surface-bound photoactive dye molecule.