ThO₂---Er₂O₃ systems containing 3, 6, 9 and 12 mol% Er₂O₃ were found to be solid solutions by XRD techniques. The lattice parameters (a) were obtained by various diffraction-angle analysis methods, and these values increased with increasing dopant level. The residual factors (R) obtained from the intensity analysis were 0.018–0.162 for the oxygen vacancy model. The results of DTA, TGA and DTG showed that no phase transition occurred in the measured temperature range. The electrical conductivities were measured as a function of temperature from 520 to 1100°C and of oxygen partial pressure from 1 × 10⁻⁴ to 2 × 10⁻¹ atm. The electrical conductivities increase with increasing temperature, and show a maximum with increasing dopant level. The calculated activation energies (E_a) decrease with increasing dopant level, but an abrupt increase in E_a appears in the 12 mol% sample. The phenomenon of the maximum in the conductivity occurring simultaneously with the minimum in the E_a can be explained by the ordering of free dopant ions and charged associates. The exponential dependence of the conductivity on the oxygen pressure (σ ∞ P_O2) is , and the main defect is an oxygen vacancy for oxygen pressures above 10⁻³ atm. Below 10⁻³ atm, the conductivity no longer has linearity. The conduction mechanism for this system is mixed ionic and electronic conduction due to oxygen ions and holes.