Summary: The diffusion of 239Pu in sintered pellets of UO2 (density 10.6) has been measured with the methods of α radiation absorption and sectioning. The temperature dependence of the diffusion coefficient is represented by the equation: D = 0.34 exp (-97300 ± 4100/RT) cm2 sec-1. Comparison has been made with the self-diffusion of Uranium by a new technique of differential diffusion measurements. It results that the activation energy for Plutonium diffusion is 6.0 kcals less than that of Uranium diffusion.