In the present work erbium and thulium molybdates Ln5.5MoO11.25−δ (Ln = Er and Tm) with fluorite and Ln6MoO12−δ (Ln = Er and Tm) with bixbyite structure have been studied. The materials have been obtained by mechanical activation method followed by sintering at 1600°С for 3 h. New compounds have been characterized by X-ray diffraction. The total conductivity was investigated using impedance spectroscopy method in dry and wet air. Oxygen diffusivity data was acquired by oxygen isotope exchange with C18O2. The combination of different techniques allowed us to determine ionic conductivity components in these compounds. Er and Tm fluorites and bixbyites showed oxygen-ion conductivity in dry air and oxygen-ion and proton conductivity in wet air up to 550–600°С. In wet atmosphere Er and Tm fluorites and bixbyites have total conductivity of about 2∙10−6 S/cm at 500 °C. At higher temperatures they are mixed oxygen-electronic conductors in dry and wet atmosphere. At lower (T < 400 °C) temperatures bixbyites are slightly better ionic conductors compared to fluorites. A high oxygen-ion mobility in all compounds above 200°С has been confirmed by isotope exchange method with С18O2: tracer diffusion coefficient values were ~10−11 – 10−10 cm2/s at 700 °C. Fluorites were demonstrated to have a higher oxygen mobility compared to bixbyites; the effect is more pronounced for Tm molybdates.