In an oxidizing environment, the oxide formed on plutonium (Pu) metal is composed of a plutonium dioxide (PuO2) top layer and a thin cubic plutonium sesquioxide (Pu2O3) middle layer. In a reducing environment, the PuO2 layer auto-reduces to cubic Pu2O3. The speed and extent of this conversion depend on the combination of temperature and time. While PuO2 provides a strong diffusion barrier against unwanted Pu corrosion by gaseous species (like hydrogen), Pu2O3 does not, since its crystal structure has chains of oxygen vacancies. The kinetics of the PuO2 reduction are, therefore, of fundamental interest and enable researchers to better protect Pu from corrosion. In this report, the oxygen-diffusion-limited kinetics of the dioxide to sesquioxide conversion were obtained by dynamically heating a PuO2-covered Pu sample from 294 to 418 K in a high-vacuum vessel equipped with an in situ spectroscopic ellipsometer. The physical/chemical constraints in the conversion process were combined with the ellipsometry method of multi-sample analysis to track the percentage of PuO2 and to compute the extent of Pu2O3 formation. The resulting diffusion coefficients were compared against and then combined with complementary literature data to produce a comprehensive set of kinetic parameters for reliably modeling oxide conversion over a larger temperature range than spanned by prior studies. The extracted thermal activation energy barrier (43.7 kJ/mol) and pre-exponential factor (5.0 × 10-10 cm2/s) for the oxygen-diffusion-limited process can be used to accurately model the PuO2 to Pu2O3 transformation in vacuum and/or inert gas applications.