The low-temperature oxidation of approximate to 10 nm diameter copper nanocrystals is characterized using in situ UV-vis absorbance spectroscopy and observed to lead to hollow copper oxide shells. The kinetics of the oxidation of solid Cu nanocrystals to hollow Cu2O nanoparticles is monitored in real-time via the localized surface plasmon resonance response of the copper. A reaction-diffusion model for the formation of hollow nanoparticles is fit to the measured time for complete Cu nanocrystal oxidation, and is used to quantify the diffusion coefficient of Cu in Cu2O and the activation energy of the oxidation process. The diffusivity measured here in single-crystalline nanoscale systems is 1-5 orders of magnitude greater than in comparable systems in the bulk, and have an Arrhenius dependence on temperature with an activation energy for diffusion of 37.5 kJ mol(-1) for 85 degrees C T 205 degrees C. These diffusion parameters are measured in some of the smallest metal systems and at the lowest oxidation temperatures yet reported, and are enabled by the unique nanoscale single-crystalline material and the in situ characterization technique.