Electrical conductivity of fluorite phases in the system Y2O3-Ta2O5-MgO has been studied. Y0.8Ta0.2O1.7 fluorite phase, having 15 mol% oxygen ion vacancies, showed the maximum electrical conductivity. Electrical conductivity of Y0.8Ta0.2O1.7 fluorite phase increased linearly with P-H2O(1/2) due to the contribution of protonic conduction in water vapour. The electrical conductivity of the fluorite phase increased more remarkably at lower temperature, 700 degrees C, than at higher temperatures, 800 degrees C and 900 degrees C. The obtained results imply that the protonic conduction greatly contributes to the electrical conductivity of the fluorite phases in the system Y2O3-Ta2O5-MgO in water vapour.