A self-consistent thermodynamic model of thorium dioxide and its dilute alloy Ce_0.05Th_0.95O_2, which are of great importance for atomic power engineering, has been constructed based on generalized Debye and Einstein models. In terms of this model, the temperature dependences of the bulk modulus, density, and heat capacity of the systems have been obtained. The anharmonicity of the acoustical and optical modes of the crystal lattice and its change in passing from ThO_2 to Ce_0.05Th_0.95O_2 has been discussed.