A four point Hebb-Wagner type polarization method is described. It allows (a) an accurate determination of the partial electronic conductivity of a mixed ionic electronic conductor, MIEC, and (b) a check on whether the ionic partial current is really suppressed. Measurements on CuBr show that it decomposes and that the ionic current is dominant. Doping CuBr reveals that CuBr is an n-type MIEC.