For the purpose of postirradiation measurement of the amount of CO formed during irradiation of oxide high-temperature reactor fuel particles, the equilibrium between CO and the various oxide phases inside the kernel has to be reestablished as closely as possible. To this end, the particles have to be heated to irradiation temperature for a certain time. Kinetic measurements have been performed on two types of porous pure UO2 kernels as well as on two types of porous carbon-diluted UO2 kernels. At 1200°C (1473 K), the CO equilibrium is reestablished after some hours in all kernel types. There seems to be no significant influence of any particle or irradiation parameter on that time.