A key characteristic of iron/iron oxide nanoparticles is that their core-shell structure evolves with exposure to environmental conditions, largely defining their fate and presumably altering their reactivity with other substances. Using nanoparticles with an Fe(0) core and predominantly magnetite shell, we have studied the structure and reactivity of these particles during short- and long-term diagenesis using microscopy, spectroscopy, electrochemical, and chemical probe methods. Chronopotentiometry shows depassivation shortly after immersion, suggesting breakdown of the oxide shell, which is consistent with a short-term increase in the rate of carbon tetrachloride reduction determined with batch experiments. Over longer time-periods (days), however, chronopotentiometry shows passivation is partially restored, XPS and XRD confirm the shell thickness increases, and carbon tetrachloride reduction rates decrease. The yield of chloroform from carbon tetrachloride reduction may increase slightly in the short-time regime, but then it declines and stabilizes, concurrent with regrowth of the magnetite-rich oxide film.