Elucidating the fundamental chemical, structural, and physical properties can shed new and unexpected light on the behavior of both natural and synthetic nanoparticles. Iron oxides and oxyhydroxides commonly occur as nanoparticles in the 3-10 nm size range and strongly impact the biogeochemical cycle of iron and other species at and near the Earth's surface. We use a range of techniques in order to elucidate the link between chemical reactivity and the physical and structural properties of iron oxide nanoparticles.  For example, we compare surface-area normalized rates of reduction and oxidation of well characterized iron oxide nanoparticles in order to determine the effect of particle size, nanoparticle composition (e.g., aluminum doping), and particle shape on reactivity. In addition, we employ a wide range of materials characterization, including magnetism, X-ray diffraction, and high-resolution transmission electron microscopy (HRTEM) to characterize both natural and synthetic iron oxide nanoparticles. Interestingly, surface-area normalized rates of reductive dissolution by hydroquinone show that smaller ferrihydrite nanoparticles (ca. 5 nm) are substantially less reactive than are larger ferrihydrite nanoparticles (ca. 7nm). Characterization using magnetic methods (e.g., Mössbauer and ), HRTEM, and X-ray diffraction suggest that the primary difference between the ferrihydrite samples is size. Results examining several different ferrihydrites, goethites, and magnetites of controlled particle size and with varying composition will be presented.