Dimensional changes in a variety of pyrolytic carbons irradiated at 900° to 1000°C and at 1200° to 1300°C show that during the irradiation, the rate at which the specimens change dimensions accelerates. The accelerations are most pronounced in dense, isotropic carbon and are accompanied by the generation of pores in the structure, which causes a volume expansion. The data obtained at the lower irradiation temperatures (∼900°C) suggest that the accelerations are due primarily to a degradation in crystal structure that enhances the instability of the crystallites. At the higher irradiation temperatures, increases in anisotropy contribute significantly to the accelerations. The volume expansions, the degradation of the crystal structure, and changes in thermal conductivity are all less for irradiations above 1200° than at 1000°C. These observations, together with the increases in anisotropy, are believed to be manifestations of the fact that the intercrystallite irradiation creep rate is substantially higher at irradiation temperatures above 1200° than at 1000°C. High creep rates provide an effective mechanism which relaxes the stresses that are generated between adjacent, misaligned crystallites during irradiation.