Abstract The thermal diffusivities of uranium and plutonium mixed oxide (MOX) fuels with 30% Pu-content were measured at temperatures from 990 to 2190K by the laser flash method. In high-temperature measurements of the stoichiometric specimen, a difference was observed between measurements during heating up and cooling down which seemed to be caused by a change of the oxygen to metal (O/M) ratio. Estimation of the O/M ratio decline by the Knudsen–Langmuir equation, which can be used to discuss evaporation behavior, suggested that the O/M ratio of the stoichiometric specimen had started to decrease at 1800K. For the O/M ratio of 1.95, that estimation showed that the O/M ratio did not change up to 2200K. It was also found from the experimental results that no significant changes of O/M ratios of specimens with O/M ratio less than 1.95 were observed.