Redistribution of plutonium has been examined in three hypo stoichiometric mixed oxide fuel pins irradiated in the fast flux of the RAPSODIE reactor. From the results of these examinations, the effective heat of transport of thermal diffusion of plutonium is estimated. The increase in central temperature is determined with plutonium distributions calculated from thermal diffusion and estimated from postirradiation examinations, respectively. In stoichiometric fuels, a temperature increase of up to 150°C must be anticipated due to plutonium enrichment by evaporation-condensation processes, while in strongly hypostoichiometric fuels the increase in central temperature, caused in this case by thermal diffusion, does not exceed 50°C for power ratings below 450 W/cm. The radial variation of the O/(U +Pu) ratio is calculated since it strongly influences the vapor pressures of the fuel component species and therefore influences the redistribution by evaporation-condensation processes.