Electrical conduction and oxygen diffusion mobility in the bixbyite (Ia3) and rhombohedral (R3) polymorphs of the Ln 6 MoO 12-Δ (Ln = Er, Tm, Yb; Δ = δ, δ1, δ2; δ1 > δ2) heavy lanthanide molybdates, belonging to new, previously unexplored classes of potential mixed (ionic-electronic) conductors, have been studied in the range of 200-900 °C. The oxygen self-diffusion coefficient in bixbyite (Ia3) Yb 6 MoO 12-δ phase estimated by the temperature-programmed heteroexchange with C 18 O 2 was shown to be much higher than that for rhombohedral (R3) RI (with large oxygen deficiency) and (R3) RII (with small oxygen deficiency) Ln 6 MoO 12-Δ (Ln = Tm, Yb; Δ = δ1; δ1 > δ2) oxides. According to the activation energy for total conduction in ambient air, 0.99, 0.93, and 1.01 eV in Er 6 MoO 12-δ , Tm 6 MoO 12-δ, and Yb 6 MoO 12-δ bixbyites, respectively, oxygen ion conductivity prevails in the range ∼200-500 °C. Oxygen mobility data for the rhombohedral Ln 6 MoO 12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) phases RI and RII indicate that the oxygen in these phases exhibits mobility at much higher temperatures, such as those above 600-700 °C. Accordingly, below 600-700 °C they have predominantly electronic conductivity. As shown by total conductivity study of Ln 6 MoO 12-δ (Ln = Er, Tm, Yb) bixbyites (Ia3) and rhombohedral phases Ln 6 MoO 12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) (R3) in dry and wet air, the proton conductivity contribution exists only in Ln 6 MoO 12-δ (Ln = Er, Tm, Yb) bixbyites up to 450-600 °C and decreases with a decreasing of the lanthanide ionic radius. The obtained data on the mobility of oxygen and the presence of proton contribution in bixbyites in the 300-600 °C temperature range make it possible to confirm unequivocally that Ln 6 MoO 12-δ (Ln = Er, Tm, Yb) bixbyites are mixed electron-proton conductors at these temperatures.