Oxygen diffusion through commercial Nafion films is investigated using a transient electrochemical reduction method. Direct experimental evidence is found for two Fickian diffusion coefficients corresponding to oxygen permeation through the hydrophilic channels and fluorocarbon matrix. The diffusion coefficient and solubility of oxygen in each phase, which controls the overall permeability, can be tuned via thermal annealing where films annealed at 160 degrees C have a substantially reduced oxygen permeability relative to the asreceived material. Films annealed at 200 degrees C show intermediate oxygen permeation to the as-received and 160 degrees C annealed samples. Differential scanning calorimetry and wide-angle X-ray scattering are employed to demonstrate that increasing crystallinity reduces the oxygen solubility and diffusion coefficient through the Nafion matrix. The two oxygen diffusion coefficients are discussed in the context of literature values which span more than an order of magnitude. It is found that the time scale of the experiments plays a substantial role in the measured diffusion coefficient with short time scale (<30 s) experiments sensing primarily diffusion through the hydrophilic channels. Longer time scale experiments (>300 s) are able to sense both modes of oxygen permeation.