A study of the ionic and electronic partial conductivity of Cr2O3 scales selectively grown on Ni-20%Cr alloy with and without yttria additions in hydrogen water vapour at 900 degrees C, was carried out by the asymmetry polarisation technique. The results revealed that there were two main regions for the temperature dependence of conductivity in the range 300-900 degrees C. Above 700 degrees C, the Cr2O3 scale was an intrinsic electronic conductor with a corresponding activation energy of 1.7 eV. Below 700 degrees C, the Cr2O3 scale was a mixed ionic and electronic conductor. The activation energy was 0.6eV for electronic conductivity and 0.3 eV for ionic conductivity. The ionic transference number decreased as the temperature or applied voltage increased. The data strongly suggest that the presence of the reactive element in the scale reduces the concentration of mobile species, thus lowering the ionic transport number.