Abstract: A series of solid solutions with high content of Tb₂O₃–(Tb_xTi_1−x)₄O_8−2x (x = 0.667–0.830) are synthesized in the Tb₂O₃–TiO₂ system via co-precipitation and/or mechanical activation. This is followed by high-temperature annealing for 4–22 h. The X-ray diffraction method showed that the fluorite structure was realized for (Tb_xTi_1−x)₄O_8−2x (x = 0.75–0.817). The solid solution Tb_3.12Ti_0.88O_6.44 (64mol% Tb₂O₃ (x = 0.78)) with a fluorite structure exhibited a maximum hole conductivity of ~22 S/cm at 600°C. To separate the ionic component of the conductivity in the electronic conductor Tb_3.12Ti_0.88O_6.44, its high entropy analogue, (La_0.2Gd_0.2Tm_0.2Lu_0.2Y_0.2)_3.12Ti_0.88O_6.44, was synthesized in which all rare-earth elements (REE) cations exhibited valency of +3. Consequently, the contribution of ionic (proton) conductivity (~7 × 10⁻⁶ S/cm at 600°C) was revealed with respect to the background of dominant hole conductivity. The proton conductivity of high-entropy oxide (HEО) (La_0.2Gd_0.2Tm_0.2Lu_0.2Y_0.2)_3.12Ti_0.88O_6.44 was confirmed by the detection of the isotope effect, where the mobility of the heavier O–D ions was lower than that of the O–H hydroxyls, resulting in lower conductivity in D₂O vapors when compared to H₂O.