Low-temperature oxidation of aqueous magnetite nanoparticles to maghemite has been monitored via the loss of near-IR optical absorbance. The kinetics closely follows the diffusion in a sphere model as suggested by previous literature reports. The temperature dependence of the diffusion constant is described by an Arrhenius equation with an activation energy of 21.0 kcal/mol. No intermediate optical spectra are observed, which confirms the extremely local nature of the optical transitions. A careful search for photooxidation establishes a small upper limit for the possible increase in the diffusion constant under illumination.