Dynamic oxygen permeation measurements at 20-70 degrees C on films cast from solutions and emulsions of three series of polyurethane (PU) cationomers based on MDI, HDI, and TDI that are the subject of Parts I and II of this series were made by Barrer’s vacuum technique. For the MDI system, films cast from solutions of ionized PU exhibit permeation coefficients P that are higher at 20-40 degrees C (below the glass transition temperature T-gh Of the hard domains) than at 50-70 degrees C (above T-gh) by factors of about 5 and 40. This is the opposite of what is observed for normal homopolymers and un-ionized PU. This phenomenon is attributed to the continuous/dispersed morphology of these films. At the low temperature, oxygen molecules diffuse through the continuous phase (soft domains) only. But at the high temperature, oxygen molecules diffuse through soft domains and subsequently through hard domains, leading to an increase in diffusion pathlength. For films cast from the PU emulsions, such a drop in P was not observed because the hard domains were partially inverted from the dispersed phase to a continuous phase that gives an interwoven morphology. Such morphology allows oxygen molecules to diffuse through soft and phase-inverted hard domains simultaneously. For the HDI and TDI systems, P and D vs. 1/T plots show no zone of discontinuity. This is attributed to a T-gh that is lower than or close to the permeation temperature.