Lattice and grain-boundary diffusions of protactinium ions in sintered ThO_2 and ThO_2-UO_2 Solid solution which has an atomic ratio of 1 : 1 were measured in the temperature range 1800-2000℃, by using ^Pa as a tracer and by the sectioning method. The data obtained were analysed in the light of the theory concerning grain-boundary diffusion based on a constant source model. The lattice diffusion coefficients D can be obtained by : D (Pa in ThO_2)=2.91×10^×exp (-75400/RT) cm^2/sec, D (Pa in ThO_2-UO_2)=1.88×10^×exp (-91800/RT) cm^2/sec. The products D・2a, peculiar to grain-boundary diffusion are : D\u27・2a (Pa in ThO_2)=6.66×10^×exp (-30600/RT) cm^3/sec, D\u27・2a (Pa in ThO_2-UO_2)=5.64×10^×exp (-59700/RT) cm^3/sec, where D\u27 and 2a are the grain-boundary diffusion coefficient and the grain-boundary width, respectively
